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Troubleshooting · Applies to: Direct expansion refrigeration system · Heat pump and air conditioning · CO2 booster (transcritical) · Liquid chiller (water or brine) · Ammonia with pump circulation (R717) · Cold rooms with several evaporators

System trips on low pressure

Low-pressure trips step by step: airflow and ice on the evaporator, low charge, blocked filter drier, expansion valve and low head pressure.

Last checked: Field mode: large buttons, questions first and the screen kept on.

The low-pressure switch or the controller stops the compressor when the suction pressure drops too low. Common causes are too little air across the evaporator, too little refrigerant, a restriction before the evaporator or an expansion valve that underfeeds [1][2]. Measure suction pressure, superheat and subcooling with the system running and the fault is quickly found.

Work on the refrigerant circuit may only be carried out by certified personnel. Isolate the power before working on electrical parts and take particular care with high pressures (CO2) and flammable refrigerants (A2L and A3). Always follow the manufacturer's instructions.

Confirm the fault first

Confirm that it really is the low pressure that stops the system: read the alarm log and measure the suction pressure with a gauge when the compressor stops, and compare with the pressure switch cut-out. On a pump-down system a stop on low pressure after the liquid solenoid valve has closed is normal; the fault is a trip while the solenoid is open. Measure the voltage in and out across the low pressure switch: voltage in but not out means it has opened. If it opens although the pressure is above the cut-out, the switch or its setting is faulty. If there is no voltage even at the switch input, the fault is earlier in the safety chain.

📏 Measurement guide: calculate superheat and subcooling 🔢 Alarm and fault codes

Flowchart

System trips on low pressure1. Is the evaporator fan running, and isthe evaporator free of ice and dirt?A · Likely causeToo little air across the evaporator: thefan is stopped or running backwards, orthe coil is iced up or dirty.→ Evaporator keeps icing upNoYes2. Is the superheat high and the liquidshort: low subcooling measured at thecondenser outlet before the filter drier,or on a system with a receiver a lowreceiver level?B · Likely causeToo little refrigerant in the system,almost always because of a leak.YesNo3. Is the superheat high but thesubcooling before the filter drier normalor high (or the receiver level normal)?C · Likely causeRestriction in the liquid line or theexpansion valve: a blocked filter drier,frozen moisture in the valve, athermostatic valve that has lost its bulbcharge or a faulty electronic valve.YesNo4. Is the condensing pressure very low,for example with cold outdoor air?D · Likely causeToo little pressure difference across theexpansion valve, so the evaporator isunderfed.YesNoE · Likely causeThe pressure switch or low-pressuretransducer is set wrong or faulty, or thecut-out is set too high.System trips on low pressure1. Is the evaporator fan running, and is theevaporator free of ice and dirt?A · Likely causeToo little air across the evaporator: the fan isstopped or running backwards, or the coil isiced up or dirty.→ Evaporator keeps icing upNoYes2. Is the superheat high and the liquid short: lowsubcooling measured at the condenser outlet beforethe filter drier, or on a system with a receiver a lowreceiver level?B · Likely causeToo little refrigerant in the system, almostalways because of a leak.YesNo3. Is the superheat high but the subcooling before thefilter drier normal or high (or the receiver levelnormal)?C · Likely causeRestriction in the liquid line or the expansionvalve: a blocked filter drier, frozen moisture inthe valve, a thermostatic valve that has lostits bulb charge or a faulty electronic valve.YesNo4. Is the condensing pressure very low, for examplewith cold outdoor air?D · Likely causeToo little pressure difference across theexpansion valve, so the evaporator is underfed.YesNoE · Likely causeThe pressure switch or low-pressure transducer isset wrong or faulty, or the cut-out is set too high.

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

  1. 1
    Flowchart B

    Too little refrigerant in the system, almost always because of a leak.

    Remedy: Leak test and repair the leak before adding refrigerant. Charge according to the manufacturer's instructions and record the quantity in the logbook. In the USA, appliances with 15 lb or more of HFC or another refrigerant with GWP above 53 must be repaired within 30 days once the leak rate exceeds 20 % (commercial refrigeration), 30 % (industrial process) or 10 % (comfort cooling and others), followed by verification tests (40 CFR 84.106) [3].

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Leak test a refrigeration system

    Find leaks systematically, confirm the repair and record the work. Work that opens the refrigerant circuit requires F-gas certification (Regulation (EU) 2024/573 or national equivalent, EPA 608 in the USA).

    1. Prepare the system and the detector. The system must be under pressure (running, or at standstill with warm ambient). Stop fans in the area to get calm air and ventilate low or enclosed spaces. With A2L/A3 refrigerants use no ignition sources and only detectors rated for them; with CO2 (R744) remember the very high pressure. Check the electronic detector (EN 14624) against its reference leak or as the manufacturer describes.

      ✓ Check: The detector reacts to the reference leak, and the air around the system is calm.

    2. Search with the electronic detector. Move the probe slowly, a few centimetres per second, close to every brazed joint, flare, valve stem, Schrader core, gauge and coil bend. Refrigerants are heavier than air, so hold the probe at and below each joint. Follow the circuit in a fixed order so that nothing is missed.

      ✓ Check: Every joint along the circuit has been passed, and each alarm has been repeated at the same spot.

    3. Confirm with bubble solution. Spray bubble leak solution on each suspected point and watch it for at least a minute; small leaks form bubbles or foam slowly.

      ✓ Check: Bubbles or foam pinpoint the leak, or the point is confirmed tight.

    4. Use UV dye only where it is approved. If the system already contains a UV dye approved by the compressor manufacturer, light the joints with a UV lamp and wear UV glasses.

      ✓ Check: A fluorescent trace marks the leak, or no trace is visible.

    5. Repair correctly. Recover the refrigerant (or isolate the section), repair with nitrogen flowing when brazing, pressure test with nitrogen, evacuate and charge by weight. Never top up refrigerant 'to test' without finding and repairing the leak.

      ✓ Check: The repaired joint holds the nitrogen test pressure and the system is charged with the specified amount.

    6. Record and re-check. Write the leak location, cause, repair, refrigerant amount and your certificate number in the equipment logbook. Check the repair again within one month, as the F-gas regulation requires.

      ✓ Check: The logbook entry is complete and a follow-up check is booked.

    Charge refrigerant by weight

    The only reliable way to get the correct charge is to weigh it in. The system must be leak tight, pressure tested and evacuated first, and the work must be done by F-gas certified personnel.

    1. Check that the system is ready. Check that leaks are repaired, the nitrogen pressure test is passed and the system is evacuated with a passed decay test (see those guides). Find the charge on the nameplate plus any addition for pipe length given by the manufacturer.

      ✓ Check: Records of the tests exist and you know the total charge in kg.

    2. Set up cylinder and scale. Check the refrigerant on the cylinder label (not by colour). Place the cylinder on a calibrated scale on a level surface, connect evacuated or low-loss hoses and zero the scale. On A2L/A3 systems: no ignition sources, ventilation and a gas detector.

      ✓ Check: The scale reads zero and the hoses are free of air.

    3. CO2 only: break the vacuum with vapour. On a CO2 system, charge vapour first until the pressure is well above the triple point (5.2 bar absolute) before any liquid goes in. Liquid into a deep vacuum turns to dry ice.

      ✓ Check: The system pressure is above 5.2 bar absolute before liquid charging starts.

    4. Charge liquid into the high side. With the system off, charge liquid into the receiver or liquid line. Always charge blends (400 series) as liquid. Watch the scale and close when the target is reached.

      ✓ Check: The scale shows the weight that has left the cylinder.

    5. Add the rest carefully. If not all of the charge goes in, start the system and add the remainder slowly into the suction side through a throttled valve, so that liquid does not reach the compressor. Never exceed the nameplate charge.

      ✓ Check: The total on the scale equals the target charge.

    6. Check operation. Run until stable and measure superheat and subcooling.

      ✓ Check: Both are within the manufacturer's targets.

    7. Disconnect and record. Close the cylinder, remove the hoses with minimal loss, refit the caps and leak test the ports. Record type and quantity charged in the logbook.

      ✓ Check: The caps are on, no leak is found and the logbook shows the charged quantity.

    Record work in the F-gas logbook

    Leak checks, repairs and every kilogram charged or recovered must be recorded. In the EU this follows Regulation (EU) 2024/573; other countries have their own rules (USA: EPA, 40 CFR Part 82 and 84).

    1. Find the right record. Open the logbook or digital register for the specific system and check that the plant ID, location and refrigerant match the nameplate.

      ✓ Check: The record belongs to the system you are working on.

    2. Record who and when. Enter the date, your company's certificate number, your own F-gas certificate number and your name.

      ✓ Check: Date and both certificate numbers are entered.

    3. Describe the work. Enter the type of work (leak check, repair, maintenance, installation, decommissioning), the result of any leak check, where a leak was found and how it was repaired.

      ✓ Check: Someone else can understand from the entry what was done and found.

    4. Record the quantities. Enter the refrigerant type and the quantity added and recovered in kg, read from the scale. For recovered refrigerant, note whether it is to be recycled, reclaimed or destroyed and where it goes. The CO2 equivalent can be added.

      ✓ Check: Quantities in kg match the scale readings.

    5. Plan the follow-up. After a leak repair, the system must be checked again by certified personnel within one month. Note that date and the next periodic leak check.

      ✓ Check: Both dates are written in the logbook.

    6. Sign and keep. Sign the entry and give the operator a copy if the logbook is not kept on site. The operator keeps the records for at least five years.

      ✓ Check: The entry is signed and the operator has access to it.

    Leak check of refrigeration systems: F-gas intervals and steps →

  2. 2
    Flowchart C

    Restriction in the liquid line or the expansion valve: a blocked filter drier, frozen moisture in the valve, a thermostatic valve that has lost its bulb charge or a faulty electronic valve.

    Remedy: Measure the temperature before and after the filter drier. A difference of more than about 2 K indicates a blockage. Check the valve's bulb, mounting and setting, and the electronic valve's driver.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Find a restriction in the liquid line

    A partly blocked filter drier, strainer or valve makes the liquid flash and starves the evaporator. Pressure drop shows up as a temperature drop at the restriction.

    1. Confirm the symptoms. In stable operation, measure superheat and subcooling. A restriction typically gives high superheat and low suction pressure while subcooling before the restriction is normal or high.

      ✓ Check: The measurements fit a restriction rather than a simple shortage of refrigerant.

    2. Look and feel along the line. Follow the liquid line from the condenser or receiver. Look for sweating or frost and feel for a cold spot right after a component. Wear gloves against cold burns.

      ✓ Check: You have found any spot that is clearly colder than the line before it.

    3. Measure across each component. Clamp thermometers before and after the filter drier, solenoid valve, strainer, sight glass and shut-off valves, one component at a time.

      ✓ Check: The temperature is the same on both sides of a free component; a noticeable drop of more than a degree or two marks the restriction.

    4. Measure the pressure drop. Where there are service ports before and after the filter drier or valve, measure the pressure on both sides and compare with the maker's permitted pressure drop.

      ✓ Check: The pressure drop is within the maker's data, or the component is confirmed as restricted.

    5. Check the sight glass and valves. Bubbles after a component but clear liquid before it point to that component. Check that hand and shut-off valves are fully open and that the solenoid valve opens fully when energised.

      ✓ Check: All valves are fully open and you know where flashing starts.

    6. Repair. Replacement opens the circuit and needs F-gas certified personnel: pump down or recover, never trap liquid between closed valves (especially CO2), braze with nitrogen flowing, pressure test, evacuate and recharge by weight.

      ✓ Check: After repair there is no temperature drop across the component and superheat is back to normal.

    Check a thermostatic expansion valve

    A TXV that sticks, has lost its bulb charge, has a blocked strainer or ice in the orifice gives high superheat and poor cooling. Check the valve's response first; open the circuit only when you know the valve must be cleaned or replaced.

    1. Measure the starting point. Connect gauges to the suction side and a clamp thermometer at the bulb position on the evaporator outlet. Let the system run stable and calculate the superheat.

      ✓ Check: Suction pressure, pipe temperature and superheat are noted with the system running stable.

    2. Warm the bulb. Loosen the bulb and warm it gently in your hand or in warm water (no flame or heat gun close to it). The valve should open.

      ✓ Check: The suction pressure rises and the superheat falls within a minute or two.

    3. Cool the bulb. Put the bulb in ice water. The valve should close.

      ✓ Check: The suction pressure falls and the superheat rises. If nothing changes in either test, the bulb charge is lost or the valve is stuck.

    4. Look for ice in the orifice. If the valve works for a while and then starves, wrap it in a warm cloth. If cooling returns, there is moisture in the system: replace the filter drier and evacuate.

      ✓ Check: Cooling returns when the valve body is warmed, or this cause is ruled out.

    5. Clean the strainer or replace the valve. Opening the circuit requires F-gas certification. Recover or pump down the refrigerant, open the inlet connection and clean the strainer, or replace the valve or power head that has lost its charge. Braze with nitrogen flowing, then pressure test with nitrogen, evacuate and charge by weight.

      ✓ Check: The strainer is clean or the new valve is fitted, and the system holds the pressure test and vacuum.

    6. Refit the bulb and check the superheat. Refit the bulb in its original position with good contact and insulate it. Run the system stable and measure the superheat again.

      ✓ Check: The superheat is stable and within the value given for the system.

    Check an electronic expansion valve

    An electronic expansion valve (EEV) is only as good as its sensors, cable and driver. Check them in that order before you blame the valve itself.

    1. Read driver alarms and valve opening. Read the alarm log, the superheat setpoint, the measured superheat and the valve opening (%) on the driver or service tool. Note whether the valve is stuck at 0 % or 100 % or hunting.

      ✓ Check: You know what the driver measures and what it is trying to do with the valve.

    2. Check the sensors. Compare the suction pressure transducer with a calibrated gauge at the same point, and the suction temperature sensor with a reference thermometer next to it. Check that the sensor is clamped tight on clean pipe and insulated.

      ✓ Check: Both readings agree with the references within the sensor's tolerance; otherwise fix the sensor first.

    3. Isolate and measure the coil. Isolate the driver supply, lock off and prove dead. Disconnect the valve plug and measure the resistance of each winding (for a typical bipolar stepper, two windings), and from each winding to the valve body.

      ✓ Check: The windings read the same as each other and match the valve data sheet; there is no connection to the body. OL means an open winding.

    4. Check cable and connector. Check the cable for damage and the connector for moisture, corrosion and loose pins. Check that cable length and shielding follow the valve maker's limits.

      ✓ Check: The cable is intact, the connector dry and tight, and the cable within the specified length.

    5. Drive the valve manually. Reconnect and switch on. With the service tool, set the valve to manual and move it in steps from closed towards open while you watch suction pressure and superheat. Do not leave it open so far that liquid reaches the compressor. On CO2 systems do not trap liquid between a closed EEV and another closed valve.

      ✓ Check: Suction pressure and superheat change with each step; no response means a stuck valve or a dead driver output.

    6. Return to reference and automatic. Let the driver initialise the valve to its reference (closed) position and return it to automatic. If the driver has a back-up module that closes the valve on power failure, check its status.

      ✓ Check: The valve homes without alarms and superheat settles at the setpoint in automatic.

    Superheat and subcooling: how to measure and interpret them, including glide →

  3. 3

    Blocked expansion valve orifice or blocked capillary tube (dirt, wax or ice).

    Remedy: Warm the valve body carefully: if the flow returns it was ice, meaning moisture in the system. Clean the strainer and replace the orifice. A blocked capillary tube is replaced together with the filter drier, and the circuit is evacuated.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Check a thermostatic expansion valve

    A TXV that sticks, has lost its bulb charge, has a blocked strainer or ice in the orifice gives high superheat and poor cooling. Check the valve's response first; open the circuit only when you know the valve must be cleaned or replaced.

    1. Measure the starting point. Connect gauges to the suction side and a clamp thermometer at the bulb position on the evaporator outlet. Let the system run stable and calculate the superheat.

      ✓ Check: Suction pressure, pipe temperature and superheat are noted with the system running stable.

    2. Warm the bulb. Loosen the bulb and warm it gently in your hand or in warm water (no flame or heat gun close to it). The valve should open.

      ✓ Check: The suction pressure rises and the superheat falls within a minute or two.

    3. Cool the bulb. Put the bulb in ice water. The valve should close.

      ✓ Check: The suction pressure falls and the superheat rises. If nothing changes in either test, the bulb charge is lost or the valve is stuck.

    4. Look for ice in the orifice. If the valve works for a while and then starves, wrap it in a warm cloth. If cooling returns, there is moisture in the system: replace the filter drier and evacuate.

      ✓ Check: Cooling returns when the valve body is warmed, or this cause is ruled out.

    5. Clean the strainer or replace the valve. Opening the circuit requires F-gas certification. Recover or pump down the refrigerant, open the inlet connection and clean the strainer, or replace the valve or power head that has lost its charge. Braze with nitrogen flowing, then pressure test with nitrogen, evacuate and charge by weight.

      ✓ Check: The strainer is clean or the new valve is fitted, and the system holds the pressure test and vacuum.

    6. Refit the bulb and check the superheat. Refit the bulb in its original position with good contact and insulate it. Run the system stable and measure the superheat again.

      ✓ Check: The superheat is stable and within the value given for the system.

    Replace a filter drier

    Replace the filter drier after moisture, a compressor burnout, a restriction across the drier or whenever the circuit has been opened. Work on the refrigerant circuit requires F-gas certification; brazing is hot work.

    1. Isolate or recover the refrigerant. Pump down or close valves to isolate the drier section where the system allows, otherwise recover the refrigerant with a recovery unit into a marked recovery cylinder. Break the vacuum with dry nitrogen to just above atmospheric pressure before opening. With A2L/A3 refrigerants purge with nitrogen and check with a gas detector.

      ✓ Check: The section reads slightly above 0 bar(g) on nitrogen and the pressure does not rise (no refrigerant left).

    2. Choose the right drier. Use a drier of the same size or larger, with connections and capacity for the refrigerant. After a burnout or acid in the oil use a burnout (acid-removing) type and plan to replace it again after the clean-up; after moisture choose one with ample water capacity.

      ✓ Check: The new drier matches the refrigerant, size and situation, and its arrow follows the flow direction.

    3. Cut out the old drier. Cut the pipes with a pipe cutter (never a hacksaw, which leaves filings) and deburr with the pipe end held downward. Keep the new drier capped until just before fitting.

      ✓ Check: Pipe ends are round, clean and burr-free.

    4. Braze with nitrogen flowing. Get a fire permit, have an extinguisher at hand and remove flammable material. Purge a low flow of dry nitrogen through a regulator while brazing, and cool the drier body with a wet cloth so the desiccant is not overheated. Never use oxygen or compressed air. Flare driers are fitted with torque spanners instead.

      ✓ Check: Clean joints with full penetration, and no black oxide scale inside the pipe ends.

    5. Pressure test with nitrogen. Pressurise with dry nitrogen through a regulator to the test pressure for the system, never above PS, and leak test the new joints.

      ✓ Check: The pressure holds and no leak is found at the joints.

    6. Evacuate, charge and record. Evacuate with a micron gauge to the manufacturer's level (commonly below 500 micron/0.67 mbar), close off the pump and check that the vacuum holds. Charge the specified amount by weight and record the work and refrigerant amounts in the logbook.

      ✓ Check: The vacuum holds, the charge is weighed in and the logbook entry is complete.

    Evacuate a refrigeration system

    Evacuation removes air and moisture from the circuit before charging. A good vacuum and a decay test show that the system is dry and tight; moisture and air left in the circuit give acid, ice in the valve and high pressures.

    1. Check before you start. Evacuate only after the pressure test with nitrogen has passed. Opening the circuit requires F-gas certification (EPA 608 in the USA). For flammable refrigerants (A2L/A3) use a vacuum pump rated for them and remove ignition sources.

      ✓ Check: The pressure test is passed and documented, and the pump suits the refrigerant.

    2. Connect for a fast vacuum. Connect the vacuum pump to both the high and low side with short, large-bore hoses. Remove the valve cores with a core tool if you can. Fit the micron gauge on the system side, as far from the pump as practical, so that it can be isolated from the pump.

      ✓ Check: All connections are tight and the micron gauge reads the system, not the pump.

    3. Pull the vacuum. Check the pump oil, open the valves and start the pump. Never start the compressor or megger it while the system is in vacuum.

      ✓ Check: The micron gauge reading falls steadily.

    4. Reach the target vacuum. Continue until the gauge reaches the target in the equipment or compressor manufacturer's instructions; many use 500 microns (about 0.67 mbar). Large or wet systems take longer; warming the system helps moisture to boil off.

      ✓ Check: The gauge shows the target vacuum.

    5. Do the decay test. Isolate the pump from the system and watch the micron gauge, for example for 15 minutes or the time in the instructions. A reading that rises and levels off means moisture; one that keeps rising means a leak.

      ✓ Check: The reading rises only a little and levels off within the limit in the instructions.

    6. Charge or repeat. If the decay test fails: for moisture, evacuate longer; for a leak, find and repair it and pressure test again. When it passes, refit the valve cores and charge the refrigerant by weight directly from the vacuum.

      ✓ Check: The system is charged from vacuum, and the vacuum and decay results are recorded.

  4. 4

    Faulty expansion valve: a thermostatic valve that has lost its bulb charge, or an electronic valve with a fault in motor, cable or driver.

    Remedy: Warm the bulb in your hand: if the valve does not open it has lost its charge and must be replaced. On an electronic valve: measure the windings and cable and check the signal from the driver.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Check a thermostatic expansion valve

    A TXV that sticks, has lost its bulb charge, has a blocked strainer or ice in the orifice gives high superheat and poor cooling. Check the valve's response first; open the circuit only when you know the valve must be cleaned or replaced.

    1. Measure the starting point. Connect gauges to the suction side and a clamp thermometer at the bulb position on the evaporator outlet. Let the system run stable and calculate the superheat.

      ✓ Check: Suction pressure, pipe temperature and superheat are noted with the system running stable.

    2. Warm the bulb. Loosen the bulb and warm it gently in your hand or in warm water (no flame or heat gun close to it). The valve should open.

      ✓ Check: The suction pressure rises and the superheat falls within a minute or two.

    3. Cool the bulb. Put the bulb in ice water. The valve should close.

      ✓ Check: The suction pressure falls and the superheat rises. If nothing changes in either test, the bulb charge is lost or the valve is stuck.

    4. Look for ice in the orifice. If the valve works for a while and then starves, wrap it in a warm cloth. If cooling returns, there is moisture in the system: replace the filter drier and evacuate.

      ✓ Check: Cooling returns when the valve body is warmed, or this cause is ruled out.

    5. Clean the strainer or replace the valve. Opening the circuit requires F-gas certification. Recover or pump down the refrigerant, open the inlet connection and clean the strainer, or replace the valve or power head that has lost its charge. Braze with nitrogen flowing, then pressure test with nitrogen, evacuate and charge by weight.

      ✓ Check: The strainer is clean or the new valve is fitted, and the system holds the pressure test and vacuum.

    6. Refit the bulb and check the superheat. Refit the bulb in its original position with good contact and insulate it. Run the system stable and measure the superheat again.

      ✓ Check: The superheat is stable and within the value given for the system.

    Check an electronic expansion valve

    An electronic expansion valve (EEV) is only as good as its sensors, cable and driver. Check them in that order before you blame the valve itself.

    1. Read driver alarms and valve opening. Read the alarm log, the superheat setpoint, the measured superheat and the valve opening (%) on the driver or service tool. Note whether the valve is stuck at 0 % or 100 % or hunting.

      ✓ Check: You know what the driver measures and what it is trying to do with the valve.

    2. Check the sensors. Compare the suction pressure transducer with a calibrated gauge at the same point, and the suction temperature sensor with a reference thermometer next to it. Check that the sensor is clamped tight on clean pipe and insulated.

      ✓ Check: Both readings agree with the references within the sensor's tolerance; otherwise fix the sensor first.

    3. Isolate and measure the coil. Isolate the driver supply, lock off and prove dead. Disconnect the valve plug and measure the resistance of each winding (for a typical bipolar stepper, two windings), and from each winding to the valve body.

      ✓ Check: The windings read the same as each other and match the valve data sheet; there is no connection to the body. OL means an open winding.

    4. Check cable and connector. Check the cable for damage and the connector for moisture, corrosion and loose pins. Check that cable length and shielding follow the valve maker's limits.

      ✓ Check: The cable is intact, the connector dry and tight, and the cable within the specified length.

    5. Drive the valve manually. Reconnect and switch on. With the service tool, set the valve to manual and move it in steps from closed towards open while you watch suction pressure and superheat. Do not leave it open so far that liquid reaches the compressor. On CO2 systems do not trap liquid between a closed EEV and another closed valve.

      ✓ Check: Suction pressure and superheat change with each step; no response means a stuck valve or a dead driver output.

    6. Return to reference and automatic. Let the driver initialise the valve to its reference (closed) position and return it to automatic. If the driver has a back-up module that closes the valve on power failure, check its status.

      ✓ Check: The valve homes without alarms and superheat settles at the setpoint in automatic.

  5. 5
    Flowchart A

    Too little air across the evaporator: the fan is stopped or running backwards, or the coil is iced up or dirty.

    Remedy: Check the fan and the defrost. Clean or defrost the evaporator and find out why the ice formed.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Check a condenser or evaporator fan motor

    A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

    1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

    2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

    3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

    4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

    5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

    6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

    Check the defrost system

    Incomplete defrost leaves ice on the evaporator, which reduces capacity and can block the airflow completely. Check heaters or hot gas, termination, interval, drip time and fan delay.

    1. Check the defrost settings. Read defrost type, interval, maximum time, termination temperature, drip time and fan delay in the controller and compare with the manufacturer's recommendations. Check the controller clock if defrosts are scheduled.

      ✓ Check: The settings match the application and the clock is right.

    2. Measure the heater resistance. Isolate, lock off and prove dead, then disconnect the heaters and measure each one. The expected resistance is R = U² ÷ P from the heater's voltage and power. Also measure insulation to earth with an insulation tester.

      ✓ Check: Each heater is close to the calculated value and has no earth fault; OL means a broken heater.

    3. Measure the current during defrost. Restore power, start a manual defrost and measure the current in each heater supply with a clamp meter. Watch out for hot surfaces.

      ✓ Check: The current is about P ÷ U for each heater, or matches the nameplate defrost current.

    4. Check hot gas defrost. On hot gas systems, check that the hot gas valve opens (warm pipe after the valve) and that the liquid or suction valves close as intended. Never exceed the system PS.

      ✓ Check: Hot gas reaches the evaporator and the coil warms evenly.

    5. Check termination. Check that the termination sensor sits on the coldest part of the coil, usually where ice remains longest, and that the defrost ends on temperature rather than on maximum time.

      ✓ Check: The defrost ends on temperature and the coil is completely free of ice.

    6. Check drip time and fan delay. After the defrost, watch the drip time and the evaporator fans when refrigeration restarts. Check that the drain and the drip tray heater work.

      ✓ Check: Fans stay off until the coil has reached the fan-delay setpoint, and water drains without refreezing in the tray.

    Defrost and clean an evaporator

    A heavily iced or dirty evaporator lets too little air through, so the room gets warm and the coil ices up further. Defrost it gently and clean it without damaging fins or tubes.

    1. Protect the goods and isolate. Move or cover food near the evaporator. Isolate, lock off and prove dead the fans and defrost heaters: fans can start automatically.

      ✓ Check: The meter shows 0 V on the fan and heater terminals and the fans cannot start.

    2. Melt the ice. Start a manual defrost or melt the ice with warm water at low pressure. Never chip the ice with a screwdriver or sharp tool: punctured tubes release refrigerant. Make sure the tray and drain can take the melt water.

      ✓ Check: The coil is free of ice and no tubes or fins are damaged.

    3. Clean the coil. Spray a food-safe coil cleaner on the fins, let it work for the time on the data sheet and rinse with low-pressure water in the direction of the fins. Do not use a high-pressure washer. Straighten bent fins with a fin comb.

      ✓ Check: Light passes evenly through the coil and the rinse water runs clean.

    4. Clean the drip tray and drain. Clean and rinse the drip tray and check that the drain runs freely. Sanitise surfaces that can drip onto food.

      ✓ Check: Water poured into the tray drains away quickly.

    5. Restore power and check the fans. Refit the covers, remove your lock and switch on. Check that all fans run smoothly in the right direction without touching the shroud.

      ✓ Check: All fans run quietly and blow air through the coil in the right direction.

    6. Check the result and find the cause. Watch the room temperature come down and check the next automatic defrost. If the coil ices again, check the defrost system, the door seals and the drain heater.

      ✓ Check: The room reaches its setpoint and the coil is clear after the next defrost.

    Continue with: Evaporator keeps icing up →

  6. 6

    A solenoid or shut-off valve in the liquid line does not open fully.

    Remedy: Check that the shut-off valves are fully open, that the solenoid coil gets voltage and that it pulls in.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Check valve positions and trapped liquid

    A service or shut-off valve left closed or half open gives symptoms like a restriction or a low charge, and liquid shut in between closed valves can burst the pipe when it warms up. Wear goggles and gloves and never exceed the system PS.

    1. Go through the circuit with the diagram. Follow the flow diagram and list every service valve, ball valve and king valve from the compressor through condenser, receiver, liquid line, evaporators and suction line.

      ✓ Check: You have a list of all valves and their required position in normal operation.

    2. Check the service valves. With a valve key, check that suction, discharge and liquid service valves are fully open (back-seated). Turn slightly off the back seat only while a gauge is connected. Refit the caps afterwards.

      ✓ Check: Spindles are fully open and caps are tight.

    3. Look for partly closed valves. With the system running, measure pipe temperature before and after each valve on the liquid line with a clamp thermometer. A clear temperature drop, frost or sweating after a valve means it is partly closed or restricted.

      ✓ Check: There is no noticeable temperature drop across any valve in the liquid line.

    4. Check the solenoid valves. Check that solenoid valves open on a call: the coil is energised (magnet tester or voltage on the coil) and there is flow (temperature change after the valve).

      ✓ Check: Each solenoid valve opens when it is called and closes when it is not.

    5. Never trap liquid. Before closing two valves on a liquid-filled section, pump the liquid out or make sure the section has a pressure relief path (relief valve or open path to the receiver). Never leave a liquid line closed at both ends.

      ✓ Check: Every section that can be closed in is empty or has a relief path.

    6. CO2: check standstill protection. On R744 systems, confirm that the standstill unit, the relief valves and any pressure-holding function are in service before valves are closed, because the pressure of trapped CO2 rises quickly as it warms (see the CO2 standstill guide).

      ✓ Check: The standstill pressure stays well below PS and the relief valves are open to their outlet.

    Check a solenoid valve (liquid line or hot gas)

    A solenoid valve that does not open starves the evaporator (low pressure, warm room); one that does not close lets refrigerant through when it should not (pump-down fails, flooding, or hot gas leaking into the evaporator). Check the coil first, then the valve itself.

    1. Find out whether the valve should be open. Read in the controller or the wiring diagram when the valve is energised (for example with cooling demand, during pump-down or during hot gas defrost). Compare with what the system does right now.

      ✓ Check: You know whether the valve should be open or closed at this moment.

    2. Check that the coil gets voltage. Measure the voltage across the coil terminals when the valve should be open. Hold a steel screwdriver blade near (not touching) the top of the coil: a live coil pulls it slightly. Check that the coil voltage on its label matches the supply.

      ✓ Check: The coil has the rated voltage (label) when the valve should be open; if not, the fault is upstream: thermostat, controller output, contactor, fuse or wiring.

    3. Check the coil itself. Isolate and lock off, prove dead, and disconnect the coil. Measure its resistance and compare with the manufacturer's value; look for a burnt smell, discolouration or a cracked coil. Make sure the coil is the right type for the valve body and sits fully down on the armature tube with its nut tight.

      ✓ Check: Coil resistance matches the data (an open circuit or a very low value means a faulty coil), and the coil is the right one and correctly fitted.

    4. Check that the valve opens. With the coil energised and the system running, measure the temperature on both sides of the valve with clamp thermometers. An open valve has (almost) the same liquid temperature on both sides; a clear temperature drop or frost after the valve means it is not opening fully or is restricted. Check the flow direction arrow on the valve body.

      ✓ Check: Little or no temperature difference across the valve when it should be open, and it is fitted in the direction of the arrow.

    5. Check that the valve closes. De-energise the coil (or let the controller close it) and watch what happens: on a pump-down system the suction pressure must fall and the compressor must stop on the low pressure switch and stay off; a hot gas valve must leave the line after it cool during the freeze or cooling cycle.

      ✓ Check: The system pumps down and stays down, or the line after a hot gas valve stays cool; a valve that leaks through must be repaired or replaced.

    6. Repair or replace. A faulty coil can be changed without opening the circuit. If the valve body leaks or sticks, opening the circuit is work for certified personnel: recover the refrigerant, replace the valve (or its internal parts) with nitrogen flowing while brazing, pressure test with dry nitrogen, evacuate and charge by weight. Protect the valve from heat while brazing as the manufacturer describes.

      ✓ Check: The valve opens and closes as it should, there is no leak at the new joints, and the work is recorded in the logbook where the refrigerant rules require it.

  7. 7
    Flowchart D

    Too little pressure difference across the expansion valve, so the evaporator is underfed.

    Remedy: Check the head pressure control: fan control, speed control or head pressure valve.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Check head pressure control

    Too low condensing pressure in winter starves the expansion valve; too high in summer costs energy and trips the HP switch. Check fan control, head pressure valves and water regulating valves against the setpoint.

    1. Read the condensing pressure and setpoint. Connect a calibrated gauge to the discharge or liquid line port and note the condensing pressure setpoint and fan stages in the controller or speed controller. Never exceed the system PS.

      ✓ Check: You know the actual condensing pressure and the setpoint.

    2. Check fan staging and speed control. Watch the fans as the pressure rises and falls: stages cut in and out, or the speed changes, around the setpoint. Measure the speed controller output if a fan does not respond.

      ✓ Check: Fans start, stop or change speed at the set pressures.

    3. Check the fans with power off. If a fan does not run, isolate and lock off, check the fan motor and blades and that the coil is clean before blaming the control. Fans can start automatically.

      ✓ Check: Fans turn freely and the condenser is clean.

    4. Check head pressure valves in cold weather. On systems with head pressure valves (e.g. Sporlan ORI/ORD or Danfoss KVR/NRD), check during low ambient that the bypass to the receiver gets warm when the valve regulates, and that there is enough refrigerant for the flooded condenser (winter charge, sight glass full).

      ✓ Check: The condensing pressure is held at the valve setting and the sight glass stays full in cold weather.

    5. Check the water regulating valve. On water-cooled condensers, check that the water regulating valve opens more as the condensing pressure rises and closes at standstill. Compare water in and out temperatures.

      ✓ Check: The valve modulates and the condensing pressure is steady; no water flows at standstill.

    6. CO2 gas coolers. On transcritical CO2 (R744) the high pressure is set by the high pressure valve and the gas cooler fans together; check it as described in the CO2 high pressure valve guide.

      ✓ Check: Gas cooler outlet temperature and high pressure follow the controller setpoints.

  8. 8

    Low load: the room is already cold, the setpoint is very low or the evaporator fans cycle off, so the suction pressure falls to the cut-out.

    Remedy: Compare the room temperature with the setpoint and check thermostat, fan and capacity control. Do not lower the cut-out below the manufacturer's value to hide the problem.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Check controller settings and alarm log

    Many faults that look like hardware faults are wrong settings or alarms that keep repeating. Read the alarm log and compare the settings with the manufacturer's parameter list before you change anything.

    1. Read the alarm log. Open the alarm menu with the keypad or service tool and note every recent alarm code with date and time. Look up each code in the manufacturer's manual.

      ✓ Check: You have a list of alarms with times and know what each code means.

    2. Check the clock. Check the controller's date and time. A wrong clock shifts scheduled defrosts and night setback and makes the alarm log misleading.

      ✓ Check: The controller clock shows the correct local date and time.

    3. Record all settings before changing anything. Write down, photograph or export setpoints, differentials, neutral zone, minimum run and off times, anti-short-cycle timer, defrost interval and duration, and the operating mode.

      ✓ Check: The service report holds a complete copy of the original settings.

    4. Compare with the parameter list. Compare each value with the manufacturer's parameter list and the plant documentation. Look in particular for a differential that is too small (short cycling), timers set to zero and setpoints that do not match the application.

      ✓ Check: Every deviation is noted with the original and the recommended value.

    5. Check the safety limits. Compare low and high pressure cut-outs and temperature limits in the controller with the manufacturer's values. Never set a safety limit beyond the manufacturer's values, and the high pressure cut-out must stay below the safety valve set pressure.

      ✓ Check: All safety limits are within the manufacturer's values.

    6. Change one value at a time and test. Correct one setting at a time, check the operating mode, and let the system run long enough to see the effect. Record every change in the service report.

      ✓ Check: The system runs as expected and the alarm does not return.

    Check a condenser or evaporator fan motor

    A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

    1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

    2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

    3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

    4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

    5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

    6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

    Check capacity control

    Capacity control makes the plant follow the load. If it does not work, suction pressure swings, compressors short cycle and wear out.

    1. Read the controller settings. Note the suction pressure (or temperature) setpoint, neutral zone, number of steps, speed range and the minimum run and off timers. Compare the timers with the compressor maker's limit for starts per hour.

      ✓ Check: Settings are noted and the timers respect the maker's start limit.

    2. Check the control sensor. Compare the suction pressure transducer that the controller uses with a calibrated gauge at the same point.

      ✓ Check: The controller reading agrees with the gauge.

    3. Follow the control over time. Log suction pressure, capacity steps or speed and compressor starts over at least 30 minutes, or read the controller's trend log.

      ✓ Check: Suction pressure stays within the neutral zone and steps change in order without rapid on-off.

    4. Test each step in manual mode. In the controller's manual or test mode, switch each unloader or capacity solenoid. Measure coil voltage with a CAT III/IV meter and watch current and suction pressure. Check in the manufacturer's data whether an energised coil loads or unloads.

      ✓ Check: Each step gives a clear change in current and suction pressure in the expected direction.

    5. Check speed and digital control. For an inverter compressor, check that the speed follows the controller signal. For a digital scroll, check the modulation signal and the solenoid.

      ✓ Check: Speed or modulation follows the demand over the whole range.

    6. Count starts. Count compressor starts per hour from the log and compare with the manufacturer's maximum.

      ✓ Check: Starts per hour are below the manufacturer's limit.

  9. 9

    Electrical fault in the safety chain: a loose cable or terminal to the pressure switch, or a faulty switch that opens although the pressure is normal.

    Remedy: Tighten the terminals, check the cables and measure across the switch in operation. Replace a switch that opens at the wrong pressure.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Test a pressure switch or transducer

    High and low pressure switches protect the compressor and the system. Check that they cut out and cut in at their settings against a calibrated gauge, and that transducers send the correct signal. Never bypass a pressure switch.

    1. Connect a calibrated gauge. Connect a calibrated gauge or digital manifold to a service port on the same side and as close as possible to the switch or transducer. Wear goggles and gloves; with A2L/A3 refrigerants keep ignition sources away.

      ✓ Check: The gauge reads a stable pressure close to the switch.

    2. Read the settings. Note cut-out, cut-in (or differential) and reset type (automatic or manual) from the switch scale or the controller. The HP setting must be below the safety valve set pressure and the system PS.

      ✓ Check: You know what each switch should do and the HP setting is below the safety valve pressure.

    3. Test the low pressure switch. Slowly close the liquid line valve (pump down) while watching the gauge and the switch contact or the controller input. Do not let the compressor run into deep vacuum.

      ✓ Check: The switch opens at its cut-out setting within the manufacturer's tolerance and closes again at its cut-in.

    4. Test the high pressure switch. Where it can be done safely, reduce the condenser airflow step by step while watching the gauge. Stop immediately if the pressure reaches the setting without a trip. On CO2 systems, or where raising the pressure is not safe, test the switch off the system with nitrogen and a regulator instead.

      ✓ Check: The switch trips at its setting within tolerance and well below the safety valve pressure.

    5. Check the transducer signal. Measure the output (e.g. 0.5–4.5 V or 4–20 mA) at the controller input and convert it with the transducer range to pressure.

      ✓ Check: The calculated pressure agrees with the reference gauge within the transducer's tolerance.

    6. Restore and record. Open the valves fully, restore airflow, reset any manual reset device and write the measured cut-out and cut-in points in the service report. Replace a switch that does not trip at its setting.

      ✓ Check: The system runs normally and the measured values are documented.

  10. 10
    Flowchart E

    The pressure switch or low-pressure transducer is set wrong or faulty, or the cut-out is set too high.

    Remedy: Compare the cut-out with a calibrated gauge and with the manufacturer's value. Replace a faulty switch or transducer.

    🛠 How do I do it? Step by step

    Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

    Test a pressure switch or transducer

    High and low pressure switches protect the compressor and the system. Check that they cut out and cut in at their settings against a calibrated gauge, and that transducers send the correct signal. Never bypass a pressure switch.

    1. Connect a calibrated gauge. Connect a calibrated gauge or digital manifold to a service port on the same side and as close as possible to the switch or transducer. Wear goggles and gloves; with A2L/A3 refrigerants keep ignition sources away.

      ✓ Check: The gauge reads a stable pressure close to the switch.

    2. Read the settings. Note cut-out, cut-in (or differential) and reset type (automatic or manual) from the switch scale or the controller. The HP setting must be below the safety valve set pressure and the system PS.

      ✓ Check: You know what each switch should do and the HP setting is below the safety valve pressure.

    3. Test the low pressure switch. Slowly close the liquid line valve (pump down) while watching the gauge and the switch contact or the controller input. Do not let the compressor run into deep vacuum.

      ✓ Check: The switch opens at its cut-out setting within the manufacturer's tolerance and closes again at its cut-in.

    4. Test the high pressure switch. Where it can be done safely, reduce the condenser airflow step by step while watching the gauge. Stop immediately if the pressure reaches the setting without a trip. On CO2 systems, or where raising the pressure is not safe, test the switch off the system with nitrogen and a regulator instead.

      ✓ Check: The switch trips at its setting within tolerance and well below the safety valve pressure.

    5. Check the transducer signal. Measure the output (e.g. 0.5–4.5 V or 4–20 mA) at the controller input and convert it with the transducer range to pressure.

      ✓ Check: The calculated pressure agrees with the reference gauge within the transducer's tolerance.

    6. Restore and record. Open the valves fully, restore airflow, reset any manual reset device and write the measured cut-out and cut-in points in the service report. Replace a switch that does not trip at its setting.

      ✓ Check: The system runs normally and the measured values are documented.

Checks from the manufacturers’ manuals (35)

Values and checks the manufacturers give for their own products; they apply to the product named.

  • Danfoss EIM 365: to avoid LP trips just after start, use the start opening function: start opening degree n17 held for start time n15 keeps the valve at a fixed minimum opening after start.[4]
  • Dixell XR/XW: alarm CA from a pressure switch on the digital input (i1F = PAL) only clears when the controller is switched off and on, while CA as a serious external alarm clears when the input releases.[5]
  • NIBE F1255: the LP switch stops the compressor below about 1.5 bar and allows a restart above about 3 bar (HP switch 32 bar); the LP alarm is suppressed for one minute after each change of operating mode.[6]
  • Danfoss Optyma Plus; Carel EVD evolution: If the controller shows another suction pressure than your gauge and gives no alarm, check terminals and connector, and that the transducer range entered in the controller (min/max of full scale) matches the transducer actually fitted.[7]
  • Heatcraft Beacon II: the suction pressure shown by the board should agree with an accurate gauge within about 0.14 bar (2 psig) and the sensors with a thermometer within about 1.7 K (3 °F); the low-pressure switch time-delay relay should be set to 1 minute.[8]
  • Danfoss DHP ground source heat pumps: Ground-source heat pump: wrong antifreeze or wrong mix in the brine, or a brine alarm limit set below the freezing point of the brine, gives repeated low-pressure trips.[9]
  • Danfoss DHP ground source heat pumps: Ground collector too short (too little heat from the ground) or too long and connected in series (too high pressure drop); the brine in/out difference should stay at about 4–5 K or less, and long collectors should be split into parallel loops.[9]
  • Danfoss Optyma Plus with pump-down: the pump-down stop pressure (C33) should be the LP alarm limit (C75) plus 0.3 bar, for example 0.7 bar on low-temperature piston units or 1.7 bar on medium-temperature scroll units; C33 = 0 switches pump-down off, and units made before July 2012 lack C33 and give an LP alarm at every pump-down.[10]
  • Danfoss DHP-A/AL/C/H; NIBE F1255: Ground-source and brine heat pumps: low brine flow trips the LP switch. Check the brine strainer, air in the circuit, closed valves, a jammed pump (free the rotor through the bleed screw), wrong antifreeze type or strength (refractometer) and a collector that is too short or coils connected in series.[11]
  • Danfoss DHP heat pumps: Brine-to-water heat pump: with compressor and pumps running, a temperature difference across the brine evaporator of about 2–6 K is normal; under 1 K or over 6 K points to a blocked evaporator or wrong flow. A temperature drop above 1 K across the filter drier in operation means it is clogged.[12]
  • Temperzone UC8 with Dunan, Sanhua or Carel EEV: Electronic expansion valve that never opens: the coil is not pushed fully onto the valve body, the valve type set in the controller (DIP switches or parameter) does not match the valve fitted, or the valve cable is damaged. Swapped or wrongly placed pressure transducers give similar LP and HP faults.[13]
  • Carel EVD evolution twin: Electronic valve units that trip on low pressure right after start: the 'valve opening at start-up' is too small for the evaporator, or the low pressure protection (LOP) is disabled or set below the switch. Also confirm at least about 5 K subcooling and no bubbles ahead of the valve.[14]
  • Danfoss Optyma iCO2; general for stepper valves: Stepper expansion valves are driven past closed to find zero whenever the controller powers up, which makes a short rattling or clacking noise at the valve. If you hear nothing after switching the unit off and on, suspect the valve coil, its cable or the driver before the refrigerant circuit.[15]
  • Actron PKV variable-capacity units: the LP switch opens below 165 kPa and only resets above 330 kPa; the first two trips give 5 min off, three in a row give 15 min off (HP: cut-out 4502 kPa, reset below 3509 kPa). A unit that waits is not necessarily faulty.[16]
  • KE2 Evap OEM controller on Krack MK/MV unit coolers: Check a suction transducer by venting it to atmosphere: it should read about 0 bar(g). A reading of −1 bar (−15 psig) means an open signal wire or signal shorted to ground, or, if the wiring is fine, that the controller is set for the wrong transducer or refrigerant (psia vs psig range); full scale means the signal is shorted to the 5 V supply. Typical ranges: 0–150 psia for most HFCs, 0–300 psig R410A, 0–500 psig R744.[17]
  • LG Multi V 5 outdoor units: With the unit stopped and the pressures equalised, the high- and low-pressure sensors should show the same pressure within about 1 bar; a bigger difference means a faulty sensor. Supply is 5 V DC, output 0.5–3.5 V (LP 0–2 MPa, HP 0–5 MPa); compare with a gauge on the service valve.[18]
  • Carel EVD evolution with E2V/EEV valves: Before blaming the electronic expansion valve for low-pressure trips, confirm more than 5 K subcooling and no bubbles in the sight glass ahead of the valve. At start-up, a start-opening parameter set too low, or a driver that never receives its start signal (digital input or pLAN), keeps the valve closed and trips the LP switch.[19]
  • Hussmann Protocol: Read the suction-pressure trend in the controller log around the alarm time: a gradual fall over hours or days points to charge loss, while a low suction right after a defrost points to a solenoid valve that sticks.[20]
  • Temperzone UC8 units: Check the pressure transducer itself: about 5 V DC (±10 %) supply between supply and ground, output voltage converted with the transducer's range compared to a gauge, and look for oil inside the plug, which should be cleaned out with contact cleaner.[21]
  • ITV MR 400 ice machines with remote condenser: A remote air-cooled condenser without head pressure control holds back liquid in cold weather, so the evaporator starves and the compressor stops on low pressure. ITV rates its remote condensers for about 0 to +43 °C and reports low-pressure trips below about -3 °C outdoors.[22]
  • Carel EVD mini; Temperzone UC7 units: Electronic valve that does not open: check that the stator is pushed fully onto the valve body and wired the right way. In manual mode a full opening must make the superheat fall; if it rises, the valve turns backwards. Carel gives about 40 ohm per stator winding.[23]
  • HTPG EcoNet unit coolers; Carel EVD mini: The start opening of the electronic expansion valve is set too small, so the compressor stops on low pressure before the controller has opened the valve further, and the unit cycles at every start.[24]
  • Temperzone UC7 units: Pressure transducers can be swapped between suction and discharge, of the wrong range or set wrong in the controller, which gives false LP or HP alarms. At standstill with equalised pressures both transducers must show the same value as a gauge.[25]
  • NIBE F1255 and similar ground-source heat pumps: Ground-source heat pump: too little brine flow (blocked particle filter, air in the collector, brine pump) or a brine mix with too high freezing point.[26]
  • NIBE S1255: Ground-source heat pump: the brine temperature difference (brine in minus brine out) should be about 2–5 K with the compressor running; a larger difference means too little brine flow.[27]
  • Vaillant aroSTOR VWL B, alpha-innotec BWP 190S/260S: Exhaust or ducted air hot-water heat pump: too little air through the evaporator because the duct is too long, has too many bends or a blocked filter, or the air temperature sensor sits outside the air stream. It also gives very frequent defrosts.[28]
  • Heatcraft unit coolers with Beacon II: The distributor nozzle (orifice) after the expansion valve is the wrong size for the evaporator or refrigerant, so the evaporator is underfed.[29]
  • True Ice cubers; Actron Air ducted units: Most pressure transducers are ratiometric 0.5–4.5 V: 0.5 V means 0 bar(g) and the signal rises linearly to 4.5 V at full scale. Measure the signal wire against ground and compare with a gauge on the same point.[30]
  • Toshiba RAS split units, Actron Air Advance: Split and ducted units: the electronic expansion valve (PMV/EEV) coil should read about 46 Ω (±3–4 Ω) on each of its four windings to common at 20 °C; an open winding means a faulty coil.[31]
  • Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): The low pressure switch PS(L) opens at 0.40 MPa (4.1 kg/cm²G) and closes automatically at 0.70 MPa; SEG1 shows C1 47 or C2 47. Before looking for a leak, check the evaporator air flow: Hitachi allows only 80–120 % of nominal air flow (80–115 % on NP6 and NP8), so a clogged return air filter, a loose belt or a fan set too slow can take the suction pressure below the cut-out.[32]
  • Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: On low suction pressure VRV first opens the hot gas bypass and then makes a forced thermostat-off, with a limited number of repeats before a fault code is set; in heating a falling low pressure also lowers the compressor capacity. In cooling there is no separate low-pressure stepping-down, so a low pressure stop in cooling is reached through normal capacity control and the bypass.[33]
  • NTF linear (auger) granular ice makers, G series (G160, G280, G510, G1000): NTF G-series auger machines: the low pressure switch opens when the evaporating pressure falls below about 0.5 bar, meaning there is no heat load on the evaporator, and closes again by itself at about 2.5 bar. Look for clogged water pipes to the evaporator, a refrigerant leak, or a faulty switch.[34]
  • NTF SM flat flake ice makers (SM500–SM11000): NTF SM flakers: the low pressure safety switch is set to 0.8 bar and needs a manual reset. When it trips, check the refrigerant charge, the water level in the pump basin and that the pump runs, the pump filter and the holes of the rotating sprayer ring for scale, and that the scraper turns.[35]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II suction pressure sensor: measure between pins 2 and 3 of CN119 with the plug connected; the output runs linearly from 0.5 V at 0 MPa to 3.5 V at 1.7 MPa (about 1.76 V per MPa). Below 0.06 V for 30 s, or 5.0 V or more, gives E.86.3.[36]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II low pressure stop (P3): 0.05 MPa or less, or 0.10 MPa or less for 10 min; restart after 3 min above 0.17 MPa. Five stops within 3 h lock the unit out with E.A5.1 (power off/on and error reset needed). Earlier steps: SV2 opens at 0.20 MPa in cooling and 0.10 MPa in heating. Check in cooling the gas-side service valve, indoor EEVs and strainers, SV2, sensor and leaks; in heating the liquid-side valve, too low ambient, outdoor fan and coil, EEV1 and strainer.[37]
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Terms in this article

Low-pressure switchSuperheatSubcoolingFilter drierExpansion valve

Sources

  1. Danfoss Ref Tools – Troubleshooter: suction pressure too low
  2. Parker Sporlan – Thermostatic expansion valves: installation, field service (Bulletin 10-11) and literature
  3. 40 CFR 84.106, leak repair (USA, eCFR version of 23 October 2026)
  4. Danfoss – User guide superheat controller EIM 365, BC415322248492en-000101 (2022.09)
  5. Copeland (Dixell) – XR35CX digital controller I/O manual, 026-1203 R4 (2024)
  6. NIBE – Service manual F1255, SEM GB 1609-1 M11932 (2016)
  7. Danfoss – Dépannage sur un Optyma Plus, questions courantes (April 2021, AG302135019978fr) + Carel – EVD evolution manual +0300005EN rel. 4.2 (24.01.2024), p. 8 (parameters o20/o21) | p. 49, table 10.a
  8. Heatcraft – MAC condensing units Installation & Operation Manual, H-IM-67H (March 2018), p. 40 (preventive maintenance) and p. 42 (E7, LPS)
  9. Danfoss – Heat Pump Troubleshooting Guide (air and ground source heat pumps), VMIFB202, PDF p. 8 and p. 14
  10. Danfoss – Dépannage sur un Optyma Plus, questions courantes (April 2021, AG302135019978fr), p. 5
  11. Danfoss – DHP-A/AL/C/H service instructions VMGFC302 | NIBE – F1255 service manual, Danfoss: p. 37–38 | NIBE: p. 43–44 (alarms 51, 58)
  12. Danfoss – DHP-A/AL/C/H service instructions VMGFC302, p. 38–39
  13. Temperzone – UC8 Troubleshooting Guide, issue 2 (27 July 2015), p. 14
  14. Carel – EVD evolution TWIN manual +0300006EN rel. 2.4 (30.10.2015), p. 55, tab. 10.a
  15. Danfoss – Optyma iCO2 Application Guidelines AB399636244436en-000301 (2026.03), p. 52, 60, 63, 65 (error diagnosis)
  16. Actron Air – Variable Capacity Commercial Troubleshooting Guide PKV160T–240T, Doc. 9590-2013-03 Ver. 2 (2025-10-30), § 03.03 Fault Code Troubleshooting, E9/E11, p. 16–17
  17. Krack – MK/MV Medium Profile Unit Coolers IOM E206993_S, § 15 KE2 OEM Evap Troubleshooting Chart (PSA), p. 62
  18. LG – Multi V 5 Outdoor Unit Service Manual (09/18), Checking the High/Low Pressure Sensors, p. 122
  19. Carel – EVD evolution user manual +0300005EN rel. 4.2 (24.01.2024), p. 50, Tab. 10.a
  20. Hussmann – Protocol distributed system I&O manual P/N 365841C, pp. 6-7 – 6-8 (PDF p. 77)
  21. Temperzone – Service Training Manual, Issue 08, January 2023, pp. 283–285
  22. ITV – MR 400 Troubleshooting guide, Pressure alarm
  23. Carel – EVD mini manual +0300036EN rel. 1.2 (2018-04-23) + Temperzone – Troubleshooting Guide for UC7 controller, rev. 3 (2012-10-19), EVD mini ch. 10 pp. 32-33; UC7 p. 5 (LP)
  24. HTPG – Troubleshooting Guide EcoNet enabled unit coolers, HTPG0425_TSG (Jan 2025) + Carel – EVD mini manual +0300036EN rel. 1.2 (2018-04-23), HTPG p. 9; EVD mini p. 33
  25. Temperzone – Troubleshooting Guide for UC7 controller, rev. 3 (2012-10-19), p. 8 (F34)
  26. NIBE – F1255 Service Manual, chapter 6, Alarm 51 low pressure and Alarm 58, p. 43–45
  27. NIBE – S1255 Installer manual IHB EN 2235-2 631735, Pump adjustment, brine side
  28. Vaillant – aroSTOR VWL B installation and maintenance instructions 8000015884_00 + alpha-innotec – BWP 190S/260S operating manual OE02:961-2105, Troubleshooting (Störungsbehebung) table, p. 51–52; fault table, p. 41–42
  29. Heatcraft – MAC Series Condensing Units, H-IM-67H (March 2018), Beacon II Troubleshooting Guide, E7 low pressure safety, p. 42
  30. True Manufacturing – True Ice service manual (Spanish) TEC_TM_265 Rev. A, P#857284 (2025-08-27) + Actron Air – Advance Split Ducted Troubleshooting Guide 9590-2015-03 Ver. 3, Diagnóstico del transductor, p. 76; 04.10 Pressure-voltage table
  31. Toshiba – Service Manual Split Type, File No. SVM-18049-5 + Actron Air – Advance Split Ducted Troubleshooting Guide 9590-2015-03 Ver. 3, 11-9-4 Outdoor unit, p. 108; 04.08 EEV resistance
  32. Hitachi – Self-Contained Air Conditioners RUA-NP4ATS…RUA-NP10ATS (R410A), Installation, Operation and Maintenance Instructions Manual 65MP3197-A2 (Bosch Home Comfort Philippines); undated, PDF file created 2025, Operating range note 2 (preface); 6.5 Safety and Control Device Setting, p. 12; 3.3.6 alarm code 47, p. 7
  33. Daikin AC (Americas) – VRV Service & Troubleshooting, Participant Guide: VRV Systems Basic Operation Guide, Engineering Data EGVUSE09-07C (09/2007), with VRV III service manual navigation and Service Checker Type III workshop slides, 5.1.1 Cooling Operation and 5.1.2 Heating Operation, p. 24
  34. NTF – Technical Documentation, NTF ice makers families (SL, IC, CM, FRM/IW/DSS/ID, SLF/SLT, CVC/CV, GM/MGT, IFT, SM), 2019 (sections dated 2006–2018), Linear ice makers series – Low pressure switch, page 109
  35. NTF – Technical Documentation, NTF ice makers families (SL, IC, CM, FRM/IW/DSS/ID, SLF/SLT, CVC/CV, GM/MGT, IFT, SM), 2019 (sections dated 2006–2018), Cold flakers series – Low pressure safety pressure switch, page 150; Electric circuit, page 144
  36. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 14, p. 04-103; trouble shooting 32, p. 04-42 (undated; copy archived 2026-11-03)
  37. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, 2-6-1 Protective function list, p. 02-12; trouble shooting 45, p. 04-55 (undated; copy archived 2026-11-03)
Disclaimer: This text is guidance, not legal advice. It summarises regulations and standards and does not replace the official text. Always check the version in force and local requirements.

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International · Step-by-step guideSuperheat and subcooling: how to measure and interpret them, including glideEU · Step-by-step guideLeak check of refrigeration systems: F-gas intervals and steps
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