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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 high pressure

High-pressure trips step by step: condenser fans and dirt, recirculating warm air, overcharge, air in the system and switch faults.

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

The high-pressure switch is a safety device that stops the compressor before the pressure reaches the system's maximum allowable pressure PS. EN 378-2 requires pressure limiting on systems where the pressure could otherwise exceed PS [1]. Never reset it again and again without finding the cause.

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

Read the alarm log and measure the high pressure with a gauge when the compressor stops. Compare with the switch cut-out and the system PS. Measure the voltage in and out across the high pressure switch: voltage in but not out means it has opened. If it opens below the cut-out, the switch is faulty. Reset a manual high pressure switch only when the cause has been found.

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

Flowchart

System trips on high pressure1. Are the condenser fans (or the waterpump) running, and is the condenserclean?A · Likely causePoor heat rejection: a fan is stopped, thecondenser is clogged or the water flow istoo low.NoYes2. Is the air entering the condenserwarmer than the outdoor air, for examplebecause of recirculation or obstructions?B · Likely causeWarm air recirculates back to thecondenser, or the air inlet is blocked.YesNo3. Stop the compressor and let the fansrun until the condenser has reachedoutdoor temperature: is the pressureclearly higher than the saturationpressure at the measured condensertemperature (bubble point for blends)?C · Likely causeAir or nitrogen in the system(non-condensables), usually after poorevacuation.YesNo4. Is the subcooling unusually high (or, ona system with a receiver, the receiverlevel high)?D · Likely causeToo much refrigerant, so part of thecondenser fills with liquid.YesNoE · Likely causeThe pressure switch or transducer is setwrong or faulty, or a valve on thedischarge side is closed.System trips on high pressure1. Are the condenser fans (or the water pump)running, and is the condenser clean?A · Likely causePoor heat rejection: a fan is stopped, thecondenser is clogged or the water flow is toolow.NoYes2. Is the air entering the condenser warmer than theoutdoor air, for example because of recirculation orobstructions?B · Likely causeWarm air recirculates back to the condenser,or the air inlet is blocked.YesNo3. Stop the compressor and let the fans run until thecondenser has reached outdoor temperature: is thepressure clearly higher than the saturation pressureat the measured condenser temperature (bubble pointfor blends)?C · Likely causeAir or nitrogen in the system(non-condensables), usually after poorevacuation.YesNo4. Is the subcooling unusually high (or, on a systemwith a receiver, the receiver level high)?D · Likely causeToo much refrigerant, so part of the condenserfills with liquid.YesNoE · Likely causeThe pressure switch or transducer is set wrong orfaulty, or a valve on the discharge side is closed.

Causes, most likely first

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

  1. 1
    Flowchart A

    Poor heat rejection: a fan is stopped, the condenser is clogged or the water flow is too low.

    Remedy: Check fans, motor protection and controls. Clean the condenser. Check the pump, strainer and valves on the water side.

    🛠 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).

    Clean a condenser or gas cooler

    A dirty condenser or gas cooler raises the condensing pressure, the power consumption and the risk of high pressure trips. Clean it, straighten the fins and check that air can flow freely.

    1. Measure before you clean. With the system running, note the condensing pressure (or gas cooler outlet temperature on CO2) and the air-on temperature, so that you can see the effect afterwards.

      ✓ Check: Pressure and air temperature are recorded.

    2. Isolate and lock off. Isolate the condensing unit and fans, lock and tag the isolator and prove dead at the fan motor terminals. Fans can otherwise start automatically. On roofs, use fall protection.

      ✓ Check: 0 V at the fan terminals and the fans have stopped.

    3. Remove loose dirt. Brush off leaves and fluff from the air-entering side with a soft brush along the fins. Blow or rinse from the air-leaving side, against the normal airflow, so dirt is pushed out rather than in.

      ✓ Check: The surface of the coil is free of loose dirt.

    4. Clean with coil cleaner and water. Protect fan motors and electrical parts from water. Apply a coil cleaner suitable for the fin material, follow its safety data sheet (gloves and goggles) and rinse with low-pressure water against the normal airflow. Never use a high-pressure washer, which flattens the fins.

      ✓ Check: The rinse water runs clear and the fins are undamaged.

    5. Straighten fins and check clearances. Straighten bent fins with a fin comb of the right pitch. Check that nothing blocks the air inlet or outlet and that warm air cannot recirculate to the inlet (walls, snow, other units).

      ✓ Check: Fins are straight and the free distances around the unit match the installation instructions.

    6. Restart and compare. Remove your lock, restore power and let the system stabilise. Compare the condensing pressure and the air-on temperature with the values before cleaning.

      ✓ Check: The condensing temperature is lower at a similar air temperature.

    Check pump, strainer and flow switch on the water side

    Low water flow through the evaporator or condenser of a chiller or heat pump causes high pressure trips, freezing of the evaporator and lost capacity. Most causes are found at the pump, the strainer, air in the circuit or the flow switch.

    1. Check the pump in operation. With the unit running, measure the pump current and compare it with the nameplate. Read the differential pressure across the pump or across the heat exchanger and compare it with the design value or the pump curve. Check the rotation direction on three-phase pumps.

      ✓ Check: Current, rotation and differential pressure agree with the nameplate and design data.

    2. Isolate and lock off. Before opening anything on the water side near electrical parts, switch off the pump and the unit, lock off the isolator and prove dead at the pump terminals.

      ✓ Check: The voltage tester shows 0 V on all pump terminals.

    3. Depressurise and clean the strainer. Close the valves on both sides of the strainer and let off the pressure through a drain or vent before opening the cap: hot or glycol water under pressure can spray. Remove and clean the mesh, check the gasket and refit. Collect glycol water, do not drain it to the sewer.

      ✓ Check: The mesh is clean and undamaged, and the cap is tight with no drips after refilling.

    4. Vent the circuit. Refill to the correct static pressure and open the vents at the high points, starting with the lowest, until only water comes out. Check the pressure again and top up if it has fallen.

      ✓ Check: Each vent gives a steady stream of water without air and the system pressure is back at its setting.

    5. Check valves and the flow switch. Check that all isolating and balancing valves in the circuit are open as intended. Check that the flow switch paddle is the right length for the pipe size and sits in the flow direction, or that the differential pressure switch is set as in the unit manual.

      ✓ Check: Valves are in their working positions and the flow switch is fitted and set as the manual describes.

    6. Test the interlock. With the unit running, stop the pump (or slowly close a valve) and watch the unit. The flow switch must open and stop the compressor; the compressor must not be able to start without flow.

      ✓ Check: The compressor stops when the flow stops, and starts only when the pump is running and the flow switch has closed.

  2. 2

    The condenser fan motor, capacitor, contactor or fuse is faulty, or the fan control gives no signal.

    Remedy: Measure voltage and current on the fan motor, check capacitor, contactor and fuses, and the output signal of the fan control.

    🛠 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).

    Test the start/run capacitor and the start relay

    Faulty capacitors and start relays are common reasons why a compressor or fan motor hums, overheats or will not start.

    1. Isolate and lock off. Switch off and lock the isolator (or unplug the unit) and remove the cover of the compressor terminal box. Set the multimeter to AC volts and measure between phase and neutral, and between each phase and earth.

      ✓ Check: The meter shows 0 V AC on the incoming terminals.

    2. Photograph, discharge and disconnect the capacitor. Photograph the wiring with your phone so that you can put it back the same way. Discharge the capacitor through a resistor of about 20 kΩ, 5 W with insulated leads, held across the terminals for a few seconds. Never short it with a screwdriver: the spark damages the terminals and the capacitor. Then pull off the cables.

      ✓ Check: The meter shows 0 V DC directly across the capacitor terminals.

    3. Measure the capacitance. Set the multimeter to capacitance (µF) and measure across the terminals. Compare with the rating printed on the capacitor. Tecumseh gives the tolerances: a start capacitor −0/+20 % of its rating, a run capacitor ±10 %. A swollen or leaking capacitor, or one that reads 0 µF, is faulty.

      ✓ Check: The measured value is within the tolerance of the printed rating.

    4. Check the start relay. Pull the relay off the compressor pins and look for burn marks or a smell of burnt plastic. A current relay is tested upright: L–S open, L–M closed (Tecumseh). A PTC starter must cool for about 3 minutes before a resistance reading means anything; compare it with the value in the compressor or starter data sheet (usually a few to some tens of ohms at room temperature).

      ✓ Check: The relay contacts are as described, or the PTC reads its data-sheet value; OL (open) means the part is faulty.

    5. Replace the faulty part and test run. Fit a new part with exactly the same data: capacitance in µF, voltage rating and relay type. Mount the relay the right way up. Reconnect from your photo, refit the cover, switch on and start the unit. If capacitors or relays fail again, look for the cause: low supply voltage, short cycling or a relay of the wrong type.

      ✓ Check: The compressor starts smoothly, without humming and without the motor protection tripping.

    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.

  3. 3
    Flowchart B

    Warm air recirculates back to the condenser, or the air inlet is blocked.

    Remedy: Remove the obstructions and make sure the warm air is not drawn back in. Check the clearances in the manufacturer's installation instructions.

    🛠 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.

    Clean a condenser or gas cooler

    A dirty condenser or gas cooler raises the condensing pressure, the power consumption and the risk of high pressure trips. Clean it, straighten the fins and check that air can flow freely.

    1. Measure before you clean. With the system running, note the condensing pressure (or gas cooler outlet temperature on CO2) and the air-on temperature, so that you can see the effect afterwards.

      ✓ Check: Pressure and air temperature are recorded.

    2. Isolate and lock off. Isolate the condensing unit and fans, lock and tag the isolator and prove dead at the fan motor terminals. Fans can otherwise start automatically. On roofs, use fall protection.

      ✓ Check: 0 V at the fan terminals and the fans have stopped.

    3. Remove loose dirt. Brush off leaves and fluff from the air-entering side with a soft brush along the fins. Blow or rinse from the air-leaving side, against the normal airflow, so dirt is pushed out rather than in.

      ✓ Check: The surface of the coil is free of loose dirt.

    4. Clean with coil cleaner and water. Protect fan motors and electrical parts from water. Apply a coil cleaner suitable for the fin material, follow its safety data sheet (gloves and goggles) and rinse with low-pressure water against the normal airflow. Never use a high-pressure washer, which flattens the fins.

      ✓ Check: The rinse water runs clear and the fins are undamaged.

    5. Straighten fins and check clearances. Straighten bent fins with a fin comb of the right pitch. Check that nothing blocks the air inlet or outlet and that warm air cannot recirculate to the inlet (walls, snow, other units).

      ✓ Check: Fins are straight and the free distances around the unit match the installation instructions.

    6. Restart and compare. Remove your lock, restore power and let the system stabilise. Compare the condensing pressure and the air-on temperature with the values before cleaning.

      ✓ Check: The condensing temperature is lower at a similar air temperature.

  4. 4
    Flowchart C

    Air or nitrogen in the system (non-condensables), usually after poor evacuation [2].

    Remedy: Recover the refrigerant, evacuate according to the evacuation guide and weigh in a fresh charge.

    🛠 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.

    Recover refrigerant

    Before the refrigerant circuit is opened, the refrigerant is recovered into a cylinder, never vented. Recovery requires F-gas certification (EU 2024/573 or the national equivalent; EPA 608 in the USA) and is recorded in the logbook.

    1. Choose cylinder and equipment. Use a recovery cylinder that is marked for recovery, within its test date and used for one refrigerant only; never mix refrigerants. For flammable refrigerants (A2L/A3) use a recovery unit and cylinder rated for them and remove ignition sources. Wear goggles and gloves: liquid refrigerant causes frostbite.

      ✓ Check: The cylinder label shows the refrigerant, the test date and the maximum fill weight.

    2. Connect and evacuate the hoses. Put the cylinder on the scales. Connect the recovery unit between the system service ports and the cylinder with short hoses. Evacuate the hoses (and an empty cylinder) with a vacuum pump before you start, so no air enters the recovered refrigerant.

      ✓ Check: The hoses hold vacuum and the scales show the starting weight.

    3. Recover liquid first, then vapour. Recover liquid first if the system allows it (it goes faster), then switch to vapour recovery. Follow the recovery unit manual for the valve settings.

      ✓ Check: The system pressure falls steadily.

    4. Watch the cylinder fill. Watch the scales the whole time and stop before the maximum fill weight on the cylinder (about 80 % liquid volume). Use the unit's overfill switch if the cylinder has one. Never heat the cylinder.

      ✓ Check: The cylinder weight stays below its maximum fill weight.

    5. Reach the final recovery pressure. Continue until the system reaches the recovery pressure the recovery unit and national rules call for, and check that it does not rise again after a few minutes (refrigerant left in oil or liquid pockets).

      ✓ Check: The pressure stays at the final recovery pressure after the unit is stopped.

    6. Record. Close the cylinder valves, clear the recovery unit as its manual describes, and record the recovered quantity (by weight), the refrigerant and the cylinder number in the logbook.

      ✓ Check: The logbook shows the recovered quantity and which cylinder it went into.

    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.

    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.

    Evacuating a refrigerating system step by step: dehydration and standing vacuum test →

  5. 5
    Flowchart D

    Too much refrigerant, so part of the condenser fills with liquid.

    Remedy: Recover the excess with a recovery unit until the subcooling is normal. Never vent refrigerant.

    🛠 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.

    Recover refrigerant

    Before the refrigerant circuit is opened, the refrigerant is recovered into a cylinder, never vented. Recovery requires F-gas certification (EU 2024/573 or the national equivalent; EPA 608 in the USA) and is recorded in the logbook.

    1. Choose cylinder and equipment. Use a recovery cylinder that is marked for recovery, within its test date and used for one refrigerant only; never mix refrigerants. For flammable refrigerants (A2L/A3) use a recovery unit and cylinder rated for them and remove ignition sources. Wear goggles and gloves: liquid refrigerant causes frostbite.

      ✓ Check: The cylinder label shows the refrigerant, the test date and the maximum fill weight.

    2. Connect and evacuate the hoses. Put the cylinder on the scales. Connect the recovery unit between the system service ports and the cylinder with short hoses. Evacuate the hoses (and an empty cylinder) with a vacuum pump before you start, so no air enters the recovered refrigerant.

      ✓ Check: The hoses hold vacuum and the scales show the starting weight.

    3. Recover liquid first, then vapour. Recover liquid first if the system allows it (it goes faster), then switch to vapour recovery. Follow the recovery unit manual for the valve settings.

      ✓ Check: The system pressure falls steadily.

    4. Watch the cylinder fill. Watch the scales the whole time and stop before the maximum fill weight on the cylinder (about 80 % liquid volume). Use the unit's overfill switch if the cylinder has one. Never heat the cylinder.

      ✓ Check: The cylinder weight stays below its maximum fill weight.

    5. Reach the final recovery pressure. Continue until the system reaches the recovery pressure the recovery unit and national rules call for, and check that it does not rise again after a few minutes (refrigerant left in oil or liquid pockets).

      ✓ Check: The pressure stays at the final recovery pressure after the unit is stopped.

    6. Record. Close the cylinder valves, clear the recovery unit as its manual describes, and record the recovered quantity (by weight), the refrigerant and the cylinder number in the logbook.

      ✓ Check: The logbook shows the recovered quantity and which cylinder it went into.

    Measure subcooling

    Subcooling shows how much the liquid is cooled below its condensing temperature. Together with superheat it helps judge the charge and the condenser.

    1. Run until stable. Let the system run at steady load for at least 15 minutes, not during or just after a defrost. Check that the condenser is clean and its fans run.

      ✓ Check: Pressures and temperatures are steady.

    2. Measure the liquid pressure. Connect a gauge to the port at the condenser outlet or on the liquid line (see the guide on connecting gauges). Wear goggles and gloves.

      ✓ Check: You have a steady liquid pressure.

    3. Measure the liquid temperature. Clamp a thermometer on clean pipe at the same place as the pressure reading and insulate the probe.

      ✓ Check: You have a steady pipe temperature from the same place.

    4. Convert and calculate. Convert the pressure to saturation temperature for the refrigerant (bubble point for zeotropic blends). Subcooling = saturation temperature − measured liquid temperature.

      ✓ Check: You have a subcooling value in K.

    5. Compare with the target. Compare with the manufacturer's target for the system. In systems with a receiver, subcooling at the receiver outlet is normally small and says little about the charge. On transcritical CO2 there is no subcooling on the high side; use the gas cooler outlet temperature and pressure instead.

      ✓ Check: You know whether the value is low, normal or high for this system type.

    6. Interpret with care. Low subcooling with high superheat can mean too little refrigerant or a restriction; high subcooling with high condensing pressure can mean overcharge or non-condensables. Never add refrigerant to try: leak test first, repair, recover and weigh in the correct charge (F-gas certified personnel).

      ✓ Check: The conclusion is supported by superheat and pressures, not by subcooling alone.

  6. 6
    Flowchart E

    The pressure switch or transducer is set wrong or faulty, or a valve on the discharge side is closed.

    Remedy: Check that service and isolation valves are open. Compare the cut-out with a calibrated gauge and with the system's PS.

    🛠 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.

    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.

  7. 7

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

    Remedy: Tighten the terminals and measure across the switch in operation. Check the cut-out against a calibrated gauge and replace a faulty switch.

    🛠 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 (42)

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

  • Danfoss Optyma Plus: A17 means input DI3 has lost supply from the mechanical HP/LP switch chain; the compressor stops, the condenser fan runs at full speed, and the tripped switch must be reset by hand.[3]
  • Danfoss Optyma iCO2: an HP alarm can come from the B1 transmitter; unit off, about 5.6 kΩ supply–ground and 7.3 kΩ signal–ground mean an intact sensor; reset alarms by switching off for 5 minutes.[4]
  • Dixell XC645D: a pressure switch trip resets itself, but after PEn trips within PEi it needs a manual reset: hold DOWN for 3 s or switch the controller off and on.[5]
  • Danfoss DHP heat pumps: allow roughly 14–15 litres of system water per kW of heat output; with less volume the unit short-cycles and the high pressure switch may trip.[6]
  • Copeland Electronic Unit Controller: HPL/DLL lock out after repeated trips; once the cause is gone and minimum off time (5 min with a fixed HP switch) has passed, hold Restart 3 s twice or cut the supply.[7]
  • Copeland Electronic Unit Controller: if the fans do not run on a fan-cycling unit, the condensing temperature may be below the cut-in, the sensor wrongly mounted or the wiring harness not connected.[7]
  • Full Gauge TC-900E controllers with sensor S3 on the condenser give a first-level warning (AC1) and, at the second, higher limit (AC2), switch off the loads; a repeated AC2 points to a dirty condenser or a failed condenser fan.[8]
  • Danfoss DHP heat pumps: Heat pump, water side: a non-return valve fitted the wrong way or with a higher opening pressure than the pump can give, or timer-controlled shunt or zone valves that close off a large part of the heating system.[9]
  • Danfoss Optyma Plus condensing units: High pressure at commissioning on a unit with a microchannel condenser: the condenser holds very little liquid, so the first charge must not exceed what the receiver can hold; also check that the liquid outlet valve is open.[10]
  • Copeland EazyCool ZX condensing units: The condenser air outlet faces the prevailing wind, which pushes against the fan and lowers the airflow; fit a baffle or turn the unit. Units in a corner or stacked above each other can also recirculate warm air.[11]
  • Danfoss AK-PC 782B; Temperzone UC7: After service or commissioning, check that pressure transducers and temperature sensors are on the right pipe and the right controller input: unplug each transducer and see that the right reading drops out, and cool each sensor with freeze spray and see that the right value falls. Swapped transducers give illogical pressures and false HP or LP trips.[12]
  • ebm-papst EC fans: EC condenser fans that are switched on and off by a contactor on the supply in cyclic operation instead of through the control or enable input; this wears out the fan electronics. A fan that stands still for long outdoors should run at full speed for at least 3 hours a month.[13]
  • Danfoss EKC 326A gas cooler controller: CO2 gas cooler control: the gas cooler pressure transmitter and outlet temperature sensor must sit close to the gas cooler outlet; if they are mounted far away the controller calculates a wrong high-pressure reference.[14]
  • Danfoss DHP heat pumps: Water-heating heat pump: with compressor and circulation pump running, the heating water should rise about 3–13 K across the condenser; under 3 K or over 13 K points to a blocked condenser or wrong flow. Also look for a non-return valve fitted backwards or with too high opening pressure, and timer-controlled zone valves that close off most of the heating system.[15]
  • ebm-papst EC fans: EC condenser fans switch themselves off on overheating (dirty motor, hot surroundings, too high back pressure), undervoltage or a phase missing for more than about 5 s; they restart on their own 10–40 s after the supply is back. Read the LED flash code before replacing; a latched fault is reset by switching off for at least 25 s.[16]
  • Copeland ZX/ZXL outdoor condensing units: Wind blowing straight at the condenser air outlet slows the airflow and raises the condensing pressure; a wind baffle cures it. Units placed side by side or above each other must not draw in each other's warm air.[17]
  • Actron ASPIRE and MultiElite DS R32 splits: the HP switch opens at 4.4 MPa and reads 0 Ω when closed (measure unplugged); the condenser pipe protection stops the unit at 65 °C and restarts it below 52 °C, and above 50 °C outdoor the unit stops by design.[18]
  • Kold-Draft 60 series ice machines: On a water-cooled condenser, subcooling above about 11 K at the condenser outlet in mid-cycle means overcharge; remove refrigerant and weigh in the nameplate charge.[19]
  • Krack microchannel remote air-cooled condensers: The liquid line from the condenser does not drain freely to the receiver (a trap, a rise or a long horizontal run right at the outlet), so liquid backs up in the condenser and the head pressure rises. The line should drop about 0.6–0.9 m straight after the condenser outlet and slope down to the receiver.[20]
  • Vaillant aroSTOR VWL B 80–150: Heat pump water heater trips on high pressure because the tank is not filled and purged, or the water temperature sensor has been pulled out of its dry pocket so the heat pump keeps heating above about 75 °C.[21]
  • Goodman/Daikin DX20VC with gas furnace: On dual-fuel systems the compressor and the gas furnace running at the same time loads the indoor coil with furnace heat and drives the high pressure up; check the thermostat or controller staging.[22]
  • Actron Air Advance/Aires/VCC; Temperzone UC7 units: Heat pump in heating mode: the indoor coil is now the condenser, so a dirty return-air filter, a slow or stopped indoor fan, closed zone dampers or high duct static pressure trips the high pressure switch.[23]
  • Scotsman BC0530; Toshiba/Carrier USX Edge: Scale inside a water-cooled condenser or water heat exchanger raises the condensing pressure even when the water flow looks right; it has to be descaled chemically.[24]
  • Hoshizaki KM ice machines and Steelheart reach-ins: Stop the unit and let the pressures equalise. If they do not equalise within a few minutes there is a restriction (expansion valve, drier, strainer, check valve or a damaged line). If they equalise and the HP switch still does not reset, the switch is faulty.[25]
  • BAC Vertex evaporative condensers: Evaporative condenser: blocked spray nozzles, a stopped spray pump or scale on the coil because of too little bleed or poor water treatment. BAC sets the spray water to about 0.16 bar (2.25 psig) at the spray header and never more than 0.7 bar.[26]
  • Toshiba/Carrier USX Edge; ITV MR 400: Strong wind blowing into the fan outlet of an air-cooled unit, or direct sun on a remote condenser, pushes the condensing pressure up; fit a windbreak or baffle and shade the condenser.[27]
  • NIBE F1255/S1255 and similar water-borne heat pumps: Heat pump in heating mode: too little flow on the heating side: closed radiator or floor thermostats, a blocked particle filter, air in the system or closed valves.[28]
  • NIBE S1255: Water-borne heat pump: the temperature difference between supply and return should be about 5–10 K in heating and hot-water mode; a larger difference means too little flow on the heating side.[29]
  • Hussmann transcritical CO2 racks: CO2 gas cooler: the gas cooler outlet temperature should be only a few kelvin (typically 2–5 K) above the ambient or adiabatic pad temperature; a clearly larger approach points to a dirty coil, stopped fans or a fan control fault.[30]
  • Hussmann CO2 racks; Krack Levitor II condensers with E2 controller: Check what condenser or gas cooler fans do when the control signal or network is lost: an inverted 10–0 V signal, or the fail-safe switch on the relay board (e.g. E2 8RO, S2/S3 up = ON), should make the fans run, not stop.[31]
  • Toshiba RAS split units: Inverter outdoor unit with a DC fan that stops within seconds: switch off, unplug the motor, turn the fan by hand (it must turn freely) and measure the three windings; equal readings (e.g. about 21 Ω on Toshiba RAS) mean the motor is OK and the drive on the outdoor board is suspect.[32]
  • Fujitsu inverter split and multi-split air conditioners / heat pumps: On Fujitsu splits, high pressure protection in cooling (24) or an open pressure switch (16) usually comes from an outdoor fan that does not turn while the compressor runs, blocked air to the outdoor coil, a closed 3-way valve or overcharge. The pressure switch code clears by itself once the pressure is normal again.[33]
  • Fujitsu outdoor DC fan motor error (1b): first turn the fan by hand. Then measure at the motor connector: roughly 150–380 V DC between the red and black wires and about 15 V DC between black and white. If the voltages are missing the outdoor PCB is faulty, otherwise suspect the motor.[34]
  • Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): On all RUA-NP models the high pressure switch PS(H) opens at 4.15 MPa (42.3 kg/cm²G) and closes again automatically at 3.2 MPa; SEG1 shows C1 45 or C2 45 for the circuit concerned. The unit is rated for condenser air inlet up to 52 °C DB, and at that temperature the air entering the evaporator must not exceed 18 °C WB, so check the condenser coil, its air intake and any recirculation of warm air before suspecting the switch.[35]
  • Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: VRV handles rising high pressure in steps: first the compressor capacity is reduced (and in heating the outdoor fan and outdoor expansion valve take over), then a forced thermostat-off follows. Only when these protection stops repeat more often than the limit set for each control does the unit stop with a fault code, so a high pressure fault code means repeated events, not a single peak; look for the cause over time, for example with the Service Checker.[36]
  • NTF linear (auger) granular ice makers, G series (G160, G280, G510, G1000): NTF G-series auger machines: the high pressure switch opens at 30 bar discharge pressure and must be reset by hand. On air-cooled units check for a dirty condenser or blocked air flow, a seized fan motor, a faulty fan pressure switch or a too hot room; on water-cooled units check the condenser water valve or pressostatic valve and a scaled condenser.[37]
  • NTF linear (auger) granular ice makers, G series (G160, G280, G510, G1000): NTF G-series auger machines, water-cooled: a normally closed safety thermostat on the water condenser opens at about 66 °C when the condenser cannot get rid of the heat, and resets by itself. Causes: no water flow through the condenser, the condenser water pressure control closed, scale inside the condenser, or a faulty thermostat.[38]
  • NTF SM flat flake ice makers (SM500–SM11000): NTF SM flakers: the high pressure safety switch is set to 30 bar with manual reset. After a trip check the condenser and its air filter, that the fan motors run (air-cooled), and the water flow and the setting of the pressostatic valve (water-cooled).[39]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II high pressure stop (display P2): the discharge pressure sensor reads 4.00 MPa or more; the compressor restarts after 5 min once the pressure is 3.50 MPa or less. Three P2 stops within 60 min lock the unit out with E.A4.1. Check in cooling the liquid-side service valve, outdoor fan, coil, ambient and recirculation, EEV1 and its strainer, SV2, the sensor and overcharge; in heating the gas-side valve and the indoor EEVs and strainers.[40]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): Before P2, J-II limits high pressure step by step: in cooling SV2 opens at 3.3, 3.5 or 3.8 MPa (compressor below 25, 25–30 and above 30 rps) and the outdoor fan steps up at 3.50 MPa; in heating the outdoor fan slows at 3.30 MPa, the compressor capacity is reduced at 3.20 MPa and SV2 opens at 3.3–3.5 MPa. Pressures that hang at these levels show which protection is limiting capacity.[41]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): The J-II high pressure switch on the inverter compressor opens at 4.2 ± 0.1 MPa and closes again at 3.2 ± 0.15 MPa. If it is found open at power-on the unit shows E.86.4 (wiring or switch); if it trips 3 times within 60 min it shows E.A4.2, and then also check the check valve at the oil separator outlet of compressor 1.[42]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II discharge pressure sensor: measure between pins 2 and 3 of CN118 on the main PCB with the plug connected. The output is linear from 0.5 V at 0 MPa to 4.5 V at 5.0 MPa (0.8 V per MPa); compare with a gauge. Below 0.3 V for 30 s, or 5.0 V or more, gives E.86.1.[43]
📝 Fill in a service report

Terms in this article

High-pressure switchPS (maximum allowable pressure)SubcoolingLiquid receiver

Sources

  1. EN 378-2:2016 – Part 2: Design, construction, testing, marking and documentation
  2. HVAC School – Vacuum: moisture vs. system leak
  3. Danfoss – Dépannage sur un Optyma Plus, questions courantes, AG302135019978fr-FR0201 (April 2021)
  4. Danfoss – Application guidelines Optyma iCO2 condensing units R744, AB399636244436en-000301 (2026.03)
  5. Dixell – XC645D controller for compressor racks, 1592001831 r3.5h (14.01.2022)
  6. Danfoss – Service instructions DHP-A/AL/C/H/L, VMGFC302 (2010)
  7. Copeland – Electronic Unit Controller, AE8-1376, section 9 (2013)
  8. Full Gauge Controls – TC-900E POWER instruction manual (Ver.04), section 7 Signals; TC-900E 2HP instruction manual (Ver.06), section 7 Signals
  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. Copeland – Application guidelines EazyCool ZX A2L/A1 (Rev01), p. 46
  12. Danfoss – Testing and startup guide AK-PC 782B, BC479936513201en-000101 (2024.02) + Temperzone – Troubleshooting Guide for UC7 Controller (1012-3), p. 4 | p. 8 (F34)
  13. ebm-papst – Operating instructions W3G910-LV12-36 (item 56913-5-9970, 2025-05-22), p. 14
  14. Danfoss – EKC 326A instructions, RI8NG552 (10/2011), p. 2
  15. Danfoss – DHP-A/AL/C/H service instructions VMGFC302, p. 38–39
  16. ebm-papst – K3G310-PV69-03 operating instructions (item 57280-5-9970, 2020-09-10), p. 13–15
  17. Copeland – Application Guidelines, Outdoor refrigeration units for A2L & A1 (ZX), AGL_Unit_ZX_A2L_A1_EN Rev03, § location/installation, pdf p. 46
  18. Actron Air – ASPIRE Ducted Split (R32) Troubleshooting, Doc. 9590-2031-03 Ver. 1; Actron Air – MultiElite DS Troubleshooting, Doc. 9590-2036-03 Ver. 2, TS45–TS46 (7.20 PC 30, 7.21 PC 0A); MultiElite: corresponding PC sections
  19. Kold-Draft – 60 Series Service Manual rev 04/13/17, Troubleshooting, p. 46–47
  20. Krack – Microchannel remote air-cooled condensers I&O, PN E660159, PDF p. 19, piping section
  21. Vaillant – aroSTOR VWL B 80–150 installation and maintenance instructions 0020285091_01, Annex B, p. 22–23
  22. Goodman/Daikin – DX20VC condensing unit, Installation & Service Reference IOD-4011L, p. 19, analysis guide
  23. Actron Air – Troubleshooting Guide Advance / Aires / VCC / Genesis, Doc. 9590-2015-03 Ver. 12 + Temperzone – Troubleshooting Guide for UC7 controller, rev. 3 (2012-10-19), Actron p. 40 (E11); UC7 p. 5 (HP)
  24. Scotsman – Service manual BC0530 (March 2020) + Toshiba/Carrier – Owner's manual Universal Smart X Edge heat pump chiller, 99NA30KQAISEN421 (2025-06-21), BC0530 p. 35; USX pp. 78-79 (codes 50, 63)
  25. Hoshizaki – Service manual KM-1100MAJ(50) + Hoshizaki – Service manual Steelheart UR reach-in, KM p. 11 (HPS diagnosis); Steelheart p. 29
  26. Baltimore Aircoil – Vertex evaporative condenser O&M manual RGOMVRCA01 (2025-05-29), Start-up; section 6 Bleed rate
  27. Toshiba/Carrier – Owner's manual Universal Smart X Edge heat pump chiller, 99NA30KQAISEN421 (2025-06-21) + ITV – MR 400 Troubleshooting guide, USX p. 79 (code 66); MR 400 Pressure alarm
  28. NIBE – F1255 Service Manual, chapter 6, Alarm 51 low pressure and Alarm 58, p. 43–45
  29. NIBE – S1255 Installer manual IHB EN 2235-2 631735, Pump adjustment, brine side
  30. Hussmann – Transcritical CO2 Rack, Installation & Operation, PN 3182569_G, Liquid Injection, Table 7, p. 61
  31. Hussmann – Transcritical CO2 Rack, Installation & Operation, PN 3182569_G + Krack – Levitor II Air Cooled Condenser IOM, PN E208035_P, Gas Cooler Fan Control, p. 52; Table 37, E2 MultiFlex troubleshooting
  32. Toshiba – Service Manual Split Type, File No. SVM-18049-5, 11-10 How to judge outdoor fan motor, p. 110
  33. Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models, Wall Mounted table: codes 24 and 16
  34. Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models, Wall Mounted table: code 1b (5 + 6 flashes)
  35. 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 (preface); 6.5 Safety and Control Device Setting, p. 12; 3.3.6 alarm code 45, p. 7
  36. 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
  37. 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 – High pressure switch, page 110
  38. 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 – Water condenser safety thermostat, page 115
  39. 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 – High pressure safety pressure switch, page 151
  40. 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 43, p. 04-53 (undated; copy archived 2026-11-03)
  41. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, 2-6-1 Protective function list, high pressure protection 1–5, p. 02-11 (undated; copy archived 2026-11-03)
  42. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 15, p. 04-104; trouble shooting 33 and 44, p. 04-43 and 04-54 (undated; copy archived 2026-11-03)
  43. 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 31, p. 04-41 (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.

Read also

International · Step-by-step guideEvacuating a refrigerating system step by step: dehydration and standing vacuum testInternational · Laws and regulationsEN 378 and ISO 5149: safety classes, location classes and charge limits
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