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

System does not reach temperature

Poor cooling or heating step by step: controls and sensors, low suction pressure, a weak compressor, high load and high head pressure.

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

When the cooling or heating capacity is not enough, the cause is often simple: too little air, too little refrigerant, too much load or too high a condensing pressure. A compressor that runs continuously without the temperature dropping points to a fault in the circuit or in the compressor [1]. Measure pressures and temperatures before you change anything.

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

Measure the air (or water) temperature in and out of the evaporator and compare with the setpoint and the sensor reading. Check that the system really runs: compressor, fans and solenoid valve. Measure suction pressure, high pressure, superheat and subcooling.

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

Flowchart

System does not reach temperature1. Does the compressor run continuouslywithout the temperature dropping?A · Likely causeThe controller stops too early: wrongsetpoint, a badly placed or faulty sensor,or an alarm that limits operation.→ Compressor short cyclingNoYes2. Is the suction pressure low and thesuperheat high?B · Likely causeThe evaporator is underfed: lowrefrigerant charge or a restrictionbefore the evaporator.→ System trips on low pressureYesNo3. Is the suction pressure high but thehigh pressure low (small pressuredifference) and the compressor currentlow, although the compressor runs at fullcapacity?C · Likely causeThe compressor is pumping poorly (wornvalves or a leaking internal bypass), orthe capacity control is faulty.YesNo4. Is the load higher than normal (opendoors, warm product, damaged doorseals), or is the evaporator iced, dirty orits fan running slowly?D · Likely causeExcessive load or poor heat transfer inthe evaporator.YesNoE · Likely causeHigh condensing pressure reduces thecapacity.→ System trips on high pressureSystem does not reach temperature1. Does the compressor run continuously without thetemperature dropping?A · Likely causeThe controller stops too early: wrong setpoint,a badly placed or faulty sensor, or an alarmthat limits operation.→ Compressor short cyclingNoYes2. Is the suction pressure low and the superheat high?B · Likely causeThe evaporator is underfed: low refrigerantcharge or a restriction before the evaporator.→ System trips on low pressureYesNo3. Is the suction pressure high but the high pressurelow (small pressure difference) and the compressorcurrent low, although the compressor runs at fullcapacity?C · Likely causeThe compressor is pumping poorly (worn valvesor a leaking internal bypass), or the capacitycontrol is faulty.YesNo4. Is the load higher than normal (open doors, warmproduct, damaged door seals), or is the evaporatoriced, dirty or its fan running slowly?D · Likely causeExcessive load or poor heat transfer in theevaporator.YesNoE · Likely causeHigh condensing pressure reduces the capacity.→ System trips on high pressure

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

    The evaporator is underfed: low refrigerant charge or a restriction before the evaporator.

    Remedy: Follow the low-pressure troubleshooting.

    Continue with: System trips on low pressure →

  2. 2
    Flowchart D

    Excessive load or poor heat transfer in the evaporator.

    Remedy: Reduce the load, repair door seals and door closers, clean or defrost the evaporator and check the fan. Check that the system is sized for the load.

    🛠 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 doors, seals and the cold-room envelope

    Warm, moist air that leaks into a cold room gives ice on the evaporator, frost on the walls and a higher load. Doors and penetrations are the usual weak points.

    1. Test the inside release first. Before you work inside, check from inside that the door opens with the emergency release, even when it is locked from outside. Test the person-trapped alarm button if there is one.

      ✓ Check: The door opens from inside at once, and the alarm sounds where it should.

    2. Inspect the gaskets. Look for tears, flattened or hard sections and ice on the gaskets. Close the door on a strip of paper at several points around the frame and pull it out.

      ✓ Check: The paper is held firmly all the way around; places where it slides out easily need a new or adjusted gasket.

    3. Check closing and door alarm. Open the door and let it go: check that the closer or self-closing hinges shut and latch it. Leave it open and check that the door-open alarm sounds after its set delay.

      ✓ Check: The door closes and latches by itself, and the alarm works.

    4. Check frame and relief port heating. Feel the door frame and, on freezer rooms, the pressure relief port: they should be warm and free of ice and condensation. If not, test the heater circuit (isolate, lock off and prove dead before you touch its terminals; see the guide on heater cables).

      ✓ Check: Frame and relief port are warm and dry, and the relief port flap moves freely.

    5. Check strip curtains. Check that the strips overlap, hang down to the floor and are not missing, torn or tied aside.

      ✓ Check: The curtain closes the whole opening.

    6. Inspect panel joints and penetrations. Look for frost or ice on the inside and damp on the outside at panel joints and pipe or cable penetrations; an IR camera makes this quicker. Check that penetrations are sealed and that the vapour barrier on the warm side is intact.

      ✓ Check: No frost spots or damp patches, and every penetration is sealed.

    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.

    Compare cooling load with system capacity

    If a room never reaches its temperature, or the plant cycles all the time, compare the actual load with what the plant can deliver at the conditions you measure.

    1. Record conditions. Measure room temperature and humidity, the surrounding temperature outside the room, and the temperature of products as they come in. Note evaporating and condensing temperatures.

      ✓ Check: You have a set of values measured at the same time.

    2. Collect room and use data. Note room size, insulation type and thickness, product quantity per day and its entry temperature, door openings, people, lighting, fans and defrost.

      ✓ Check: Every load source has a value or a reasoned estimate.

    3. Calculate the load. Calculate the load per 24 hours with the Kylbehov app or a recognised method (for example the cooling load chapter in the ASHRAE Refrigeration Handbook), and divide by the planned run time per day, which leaves time for defrost.

      ✓ Check: You have the required capacity in kW at the room's design conditions.

    4. Find the actual capacity. Read evaporator capacity at the actual temperature difference between room air and evaporation, and compressor capacity at the measured evaporating and condensing temperatures, from the manufacturers' data. Note that rated capacities apply to standard conditions.

      ✓ Check: You have evaporator and compressor capacity at the real conditions; the lower one limits the system.

    5. Compare with run time. Read compressor run time and starts per hour from the controller log. Compare required and actual capacity.

      ✓ Check: Capacity below the load, or near-continuous running, shows undersizing or excess load; very short cycles and many starts show oversizing.

    6. Decide on measures. Reduce the load first (doors, curtains, product temperature, envelope), then consider more capacity or a different control strategy. Write down the comparison.

      ✓ Check: A written comparison of load and capacity with the chosen measure.

  3. 3

    The evaporator fan does not run or runs too slowly: motor, capacitor, fuse or control.

    Remedy: Measure voltage and current on the fan motor and check the capacitor and 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.

  4. 4
    Flowchart E

    High condensing pressure reduces the capacity.

    Remedy: Follow the high-pressure troubleshooting.

    Continue with: System trips on high pressure →

  5. 5
    Flowchart A

    The controller stops too early: wrong setpoint, a badly placed or faulty sensor, or an alarm that limits operation.

    Remedy: Check setpoints, sensor readings and the alarm log. Compare the sensor with a calibrated thermometer.

    🛠 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 sensor against a reference

    A temperature or pressure sensor that reads wrong makes the controller regulate wrong: wrong superheat, wrong defrost, false alarms. Compare it with a calibrated reference before changing settings or parts.

    1. Fit the reference next to the sensor. Fasten a calibrated thermometer probe beside the temperature sensor on the same pipe or in the same air stream, with heat-conducting paste and insulation. For a pressure sensor, connect a calibrated gauge to a service port at the same point in the circuit. Wear goggles and gloves when connecting to the refrigerant circuit.

      ✓ Check: The reference reading is steady for a few minutes.

    2. Compare with the controller reading. Read the live value of the sensor in the controller display or service tool at the same time as the reference.

      ✓ Check: The difference is within the tolerance given by the sensor or controller manufacturer (for a temperature sensor typically within about 1 K).

    3. Check offsets and sensor type. In the controller parameter list, check that the sensor type (e.g. NTC, PT1000, 0–10 V, 4–20 mA), the pressure range and any calibration offset match the fitted sensor. Record the original values before changing anything.

      ✓ Check: Sensor type, range and offset in the controller match the sensor data sheet.

    4. Measure the sensor resistance. Isolate and lock off the controller, disconnect the sensor and measure its resistance with a multimeter while the reference shows the temperature at the sensor. Compare with the manufacturer's resistance table. A PT1000 reads 1000 Ω at 0 °C; an ice-water bath gives a simple 0 °C check point.

      ✓ Check: The resistance matches the table value for the reference temperature; OL or 0 Ω means a broken or shorted sensor or cable.

    5. Check the sensor location and mounting. Inspect the pipe clamp or pocket: firm metal-to-metal contact, heat-conducting paste, insulation against the surrounding air and the correct position (e.g. evaporator outlet, not in the air stream of a fan).

      ✓ Check: The sensor sits firmly at the correct point and is insulated.

    6. Reconnect and confirm. Reconnect the sensor, restore power and compare the reading with the reference once more. Replace a sensor that is out of tolerance with one of the same type.

      ✓ Check: Controller and reference agree within the tolerance.

    Continue with: Compressor short cycling →

  6. 6

    The solenoid valve before the evaporator does not open, or its coil or cable is faulty.

    Remedy: Check that the coil gets voltage and pulls in, and that the valve opens (temperature before and after).

    🛠 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 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 C

    The compressor is pumping poorly (worn valves or a leaking internal bypass), or the capacity control is faulty.

    Remedy: Check the capacity control and the inverter. Compare current and pressures with the compressor data. Contact the manufacturer before replacing the compressor.

    🛠 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 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.

    Check an inverter (VFD) and read its alarms

    When a speed-controlled compressor, fan or pump will not run or trips, the inverter usually tells you why. Read its alarms first, then check supply, signals and output before you replace anything.

    1. Read and note the alarm log. Before you reset or switch off, read the active fault and the fault history on the drive keypad or service tool. Note code, time and the values stored with it (current, DC bus voltage, frequency). Look up each code in the drive manufacturer's manual.

      ✓ Check: You have a written list of codes with their meaning from the manual, and you know which fault occurred last.

    2. Check the key parameters. Compare motor data (rated voltage, current, frequency), minimum and maximum speed, ramp times and current limit with the compressor or fan manufacturer's settings list. Compressors often have a minimum speed for oil return and lubrication.

      ✓ Check: The parameters match the manufacturer's list; any differences are noted before you change them.

    3. Measure the supply voltage. With a CAT III/IV multimeter and insulated probes, measure phase to phase on L1, L2 and L3 at the drive input, at standstill and in operation. Do not wear loose clothing near open live parts.

      ✓ Check: All three voltages are within the input range in the drive manual and close to each other; a missing or low phase explains supply alarms.

    4. Check run, enable and speed signals. At the control terminals, measure the run/enable input and the speed reference (0-10 V or 4-20 mA) while the controller asks for operation. Follow the safety chain (pressure switches, motor protection) if the enable is missing.

      ✓ Check: The drive display shows a run command and the reference value that the controller sends; if not, the fault is upstream of the drive.

    5. Measure output voltage and current. With the drive running, measure U-V, V-W and W-U with a meter in low-pass (VFD) mode, and the current in each output phase with a true-RMS clamp meter. Compare with the frequency on the display and the motor nameplate.

      ✓ Check: The three output voltages and currents are balanced, and the current is within the motor rating at that speed.

    6. Isolate and wait for the DC bus. Before you disconnect or tighten anything: isolate, lock off and tag. Wait at least the discharge time on the drive label, then measure DC voltage across the DC bus terminals (+/-). Check terminals for heat damage and tighten them. If you megger the motor, disconnect the motor leads from the drive first; never insulation test through the drive.

      ✓ Check: The meter shows no dangerous DC voltage on the bus before you touch anything, and the motor test is done with the drive disconnected.

    7. Restart and watch. Reconnect, close the cover, switch on and start the unit. Follow speed, current and alarms during start and a full run cycle.

      ✓ Check: The drive ramps up to the requested speed without new alarms, and the fault log shows no new entries.

    Test whether the compressor pumps

    Low pressure difference, low current and poor capacity can mean a worn compressor, but often the cause is a bypass elsewhere. Compare measurements with the manufacturer's data and rule out other causes before you condemn the compressor.

    1. Measure in stable operation. Connect gauges to suction and discharge, and clamp thermometers on the suction line and on the discharge line close to the compressor, where the manufacturer specifies. Let the system run steadily for at least 15 minutes. Beware of the hot discharge pipe.

      ✓ Check: Pressures and temperatures are steady and noted together with the saturation temperatures.

    2. Measure current and voltage. Measure the supply voltage and the current in each phase with a true-RMS clamp meter (CAT III/IV, insulated probes, no loose clothing).

      ✓ Check: You have voltage and current for all phases; the phases are balanced.

    3. Compare with the compressor data. Enter the measured evaporating and condensing temperatures in the manufacturer's selection software or data sheet and read the expected current, capacity and discharge temperature.

      ✓ Check: Measured current and pressure difference are close to the data; clearly lower current at a low pressure difference suggests the compressor is not pumping.

    4. Rule out bypasses. Check the hot gas bypass valve, unloaders or capacity control solenoids, a digital scroll valve and any reversing valve: a pipe that is hot where it should be cold shows gas leaking back to suction.

      ✓ Check: All bypass paths are closed when they should be; otherwise repair them first and measure again.

    5. Check the internal relief valve. Many hermetic and scroll compressors have an internal pressure relief valve (IPR) between discharge and suction. If it has opened, suction pressure is high, discharge low, the shell is hot and the motor protection may trip. Find out why it opened (high pressure, blocked condenser) before restart.

      ✓ Check: The cause of an opened relief valve is found and removed, or the relief valve is ruled out.

    6. Check the valves after stop (reciprocating). On a reciprocating compressor, watch the gauges right after it stops. Discharge pressure falling fast while suction rises points to leaking discharge valves. Do not run scroll compressors into deep vacuum to test them; it damages them.

      ✓ Check: Pressures equalise slowly; fast equalisation confirms leaking valves.

    7. Decide. If the compressor still pumps too little with all other causes ruled out, plan a compressor replacement (see the guide on replacing a compressor). Record the measurements.

      ✓ Check: The decision is based on measurements compared with the manufacturer's data, and they are written down.

Checks from the manufacturers’ manuals (43)

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

  • Danfoss EKE 1: a valve that stays at or near full opening points to a blocked filter drier, flash gas from undercharge, too low condensing pressure or a valve type setting that does not match the valve.[2]
  • Danfoss Colibri ETS 12C–100C: a valve that does not move should read about 10 Ω on each of its two coils at 20 °C; it is a bipolar stepper (600 full steps) and must never be fed from mains or plain DC.[3]
  • Danfoss ETS 12.5–100: each coil should measure about 52 Ω (12 V DC motor, about 100 mA per phase); a clearly different reading points to a broken coil or cable.[4]
  • Castel solenoid coils: check the voltage at the coil against the marking; AC coils accept about ±10 %, DC coils +10/−5 %, and the valve will not open against a differential above the MOPD for that coil series.[5]
  • Dixell XW60K/XR06CX; Pego ECP202; Danfoss AK-RC 113: A faulty or wrongly set door switch input: many cold room controllers stop the fans and often the compressor while the door is open (Dixell default stops both), and temperature alarms are held back, so a switch stuck in 'open' lets the room warm up without an alarm.[6]
  • AHT plug-in cabinets: A plug-in display cabinet placed in a store that is warmer or more humid than its climate class (for example ISO 23953-2 class 3: 25 °C and 60 % RH), or in a room colder than about 16 °C, cannot hold its temperature and ices up.[7]
  • Danfoss DHP heat pumps: Heat pump: measure flow and return with a digital thermometer; the heating side should show about 7–10 K difference and the brine side no more than about 4 K. A larger brine difference points to a dirty strainer, restrictions or high pressure drop.[8]
  • Danfoss DHP heat pumps: Heat pump that heats too little in cold weather: check the outdoor sensor value against its resistance table; a cable damaged by nails or staples on its way to the sensor is a common fault.[8]
  • Danfoss DHP heat pumps; NIBE S1255: The heat pump runs mostly on the electric back-up: the operating mode is set to back-up only, the degree-minute (integral) value has reached the back-up start level, the anti-legionella interval has been shortened, or the room setpoint has been raised (on Danfoss DHP 1 K more room setpoint shifts the curve about 3 K). Also check for an alarm that blocks the compressor.[9]
  • Danfoss DHP heat pumps: Too little hot water from a heat pump: the hot-water start temperature is set too high (Danfoss DHP about 40–44 °C), tapping above about 12 l/min mixes the stratified tank, or the water heater coil is too small for the heat pump (about 7 l per kW). The temperature loss from heat pump to tap should be at most 1–2 K.[8]
  • Heatcraft unit coolers: After a defrost the unit cooler fans stay off until the coil is cold enough to reset the fan delay thermostat; if the coil never gets that cold (setting wrong or thermostat faulty), the fans never restart and the room warms up.[10]
  • Danfoss CTM multi-ejector on Hussmann CO2 racks: CO2 multi-ejector: the strainer in the ejector block must be cleaned after the first two days and again every 2–3 weeks if dirt keeps circulating; a clogged strainer or ejector cartridge cuts efficiency and capacity.[11]
  • Hillphoenix Advansor CO2 booster: Measure the pressure drop across the suction filter drier: Hillphoenix changes it at about 0.35 bar (5 psi), and after any service. A drier that cools noticeably across it is also restricting.[12]
  • Toshiba RAV-GM digital inverter split systems: When poor cooling or heating is blamed on a slow leak, top up at most 10 % of the nominal charge; if that does not help, recover everything and weigh in the full charge. Charge as liquid with a scale reading to 10 g.[13]
  • Hillphoenix multi-deck and reach-in display cases: Open display cases are rated for a store at no more than 24 °C and 55 % RH (some 27 °C). Measure the discharge air velocity at the honeycomb right after defrost with a vane anemometer and compare with the case data sheet; product above the load limit line or a dirty honeycomb breaks the air curtain.[14]
  • Hillphoenix JNRZHSA/JNRBHSA self-contained R-290 cases: Small hermetic units with propane lose a noticeable part of the charge every time gauges are connected. Connect only when needed, use the shortest possible hoses, and after work evacuate to 500 microns and weigh in the charge on the data plate.[15]
  • ActronAir ESP Platinum ducted systems: Ducted split: high external static pressure (undersized ducts, tight bends, small return grille) reduces the airflow so the unit's anti-freeze or overheat protection throttles the capacity.[16]
  • ActronAir ESP Platinum: An inverter unit modulates, so pressures and superheat say little about the charge at part load. Force full capacity by setting the set point more than 4 K below room temperature in cooling (more than 4 K above in heating) before judging or correcting the charge.[16]
  • Goodman/Daikin DX20VC: On a new split installation the liquid or gas stop valve has not been fully opened, which gives low suction pressure, high superheat and weak cooling.[17]
  • Carel EVD evolution with E2V valves: Electronic valve suspected stuck closed: measure each stator winding (about 36 Ω on Carel E2V) and, with the driver in manual, open the valve fully; if the superheat stays high the valve or its wiring is faulty.[18]
  • Goodman/Daikin DX20VC; Temperzone units with Carel Power+: Inverter drives cooled by a refrigerant plate (cooling loop or cool plate) overheat and cut the compressor speed if the plate is not clamped to the drive, lacks thermal grease, or its own expansion valve does not feed it.[19]
  • Hillphoenix and Hussmann display cases: Open display case: shelves loaded past the front edge, or goods, signs and packages over the discharge or return air grilles, break the air curtain. Cases are rated for store air of at most about 24 °C (Hillphoenix) or 27 °C (Hussmann MDN) at 55 % RH; warmer or more humid stores give warm product, frost and condensation.[20]
  • HTPG EcoNet; Danfoss EKE 100; Carel EVD mini: Read the electronic valve opening on the controller. If the valve stays at or near 100 % while the superheat is high, liquid is missing at the valve: check the sight glass, that the subcooling before the valve is above about 5 K, the solenoid valve and the valve size, before blaming the valve.[21]
  • Actron Air Advance; Toshiba multi-split: Inverter air conditioners only run at full speed when the set point is far from the room temperature (Actron: more than 4 K), and many limit the compressor speed for the first minutes after start, so test the capacity with a large difference and after a few minutes.[22]
  • Hussmann FS-OB display cases: Display case: evaporator fans fitted backwards or with the wrong blade pitch, gaps in the fan plenum or missing coil close-off panels let the air bypass the coil, so the case runs warm and the coil can freeze over.[23]
  • Hussmann and Hillphoenix display cases: Display case in the wrong place or climate: closer than about 4.5 m (15 ft) to doors, windows, fans or air-conditioning outlets, or a store above about 24 °C and 55 % RH, breaks the air curtain; the case runs warm and ices up.[24]
  • Hussmann and Hillphoenix display cases: Product stocked above the load line or over the return or discharge air grilles breaks the air curtain and starves the coil of air.[24]
  • Hoshizaki Steelheart; Beverage-Air UCF: Reach-ins: a door switch that is not pressed or is faulty keeps the evaporator fan off. On cabinets with several evaporator fans, check that all of them turn at full speed; one slow fan gives a slow pull-down.[25]
  • Actron Air Tri-Capacity split ducted: Pre-charged split units: pipe runs longer than the pre-charged length without extra charge give low subcooling and high superheat. Actron aims at 4-8 K subcooling and 2-8 K superheat in cooling; on units with interlaced coils measure with all compressors running.[26]
  • Actron Air Advance, VCC and Genesis: Inverter unit is limiting itself: protection modes for overheating, discharge temperature, condensing pressure, high suction pressure or anti-freeze reduce compressor speed; the controller shows a status code for it.[27]
  • Toshiba RAS split units: Split units with a DC indoor fan that does not run: with the motor connected, check about 310–340 V DC supply, 15 V DC control supply and a speed signal of at least 1 V DC from the board. If all are present the motor is faulty; if one is missing the indoor board is.[28]
  • Actron Air Serene DS wall splits: A 'refrigerant leak' code on a wall split (e.g. EL 0C) is often calculated, not measured: the unit flags it when the indoor coil sensor does not drop at least 1 K during the first 5 minutes of cooling. Check that the coil sensor sits in its pocket before looking for a leak.[29]
  • Fujitsu split air conditioners and heat pumps: On a Fujitsu split in cooling, both pipes at the outdoor unit should be roughly 0–10 °C and the suction line should sweat but not freeze; in heating both pipes are normally 30–60 °C. A dry suction line in cooling points to a fault, usually undercharge.[30]
  • Fujitsu indoor fan errors: the unit flags a locked fan if the speed is still zero about a minute after start, and a speed error if it reaches only about a third of the target. Check the motor plug and turn the fan by hand before replacing the PCB.[31]
  • Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): After a leak the RUA-NP must not be topped up: R410A changes composition when it leaks, so recover the charge, evacuate from both the high and low side check joints for 1–2 hours to −0.1 MPa, check that the pressure does not rise within 5 minutes, and weigh in liquid to the nameplate charge within ±0.1 kg. Factory charges per circuit: NP4 3.4 kg, NP5 4.3 kg, NP6 6.8 kg, NP8 8.0 kg, NP9 3.9 + 5.0 kg, NP10 2 × 4.8 kg.[32]
  • Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: A VRV outdoor unit does not run to a fixed suction pressure set by the load: in cooling it adjusts compressor capacity every 20 s to hold a target evaporating temperature of 6 °C (43 °F, about 139 psi absolute, 0.96 MPa abs) for R410A, 5.5 °C for R22 and 7.5 °C for R407C. A field setting moves the target by ±3 K; the lower setting is meant for long piping or an outdoor unit below the indoor units, the higher setting for short piping.[33]
  • Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: In heating the VRV compressor capacity holds a target condensing temperature of 46 °C (115 °F, about 406 psi absolute, 2.8 MPa abs) for R410A and R22, and 48 °C (118 °F) for R407C, measured with the high pressure sensor. The outdoor expansion valve then holds about 5 K (9 °F) superheat at the outdoor coil outlet, and each indoor expansion valve holds about 5 K subcooling, calculated from the outdoor condensing temperature and the indoor liquid pipe thermistor.[34]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II thermistor table B (suction TH4, outdoor coil liquid TH5, liquid TH7, sub-cooler TH8/TH9): about 27.8 kΩ at −10 °C, 16.1 kΩ at 0 °C, 4.8 kΩ at 25 °C and 1.7 kΩ at 50 °C. Table C (outdoor air TH3): about 105.4 kΩ at −20 °C, 33.6 kΩ at 0 °C, 10.0 kΩ at 25 °C and 3.6 kΩ at 50 °C. A sensor that reads wrong without being open or short skews EEV, defrost and capacity control.[35]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II indoor room thermistor: about 33.6 kΩ at 0 °C, 20.1 kΩ at 10 °C, 10.0 kΩ at 25 °C and 5.3 kΩ at 40 °C; indoor coil inlet/outlet thermistors: about 168.6 kΩ at 0 °C, 62.5 kΩ at 20 °C and 49.8 kΩ at 25 °C. Both inputs on the indoor PCB give 5 V DC with the sensor unplugged.[36]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II runs oil recovery after 3 h of compressor running in cooling and 8 h in heating (the first time after 1 h). Cooling oil recovery lasts until suction superheat is 5 K or less (at least 30 s, at most 6 min); heating oil recovery lasts 4 min. Meanwhile the outdoor display shows "or" and the indoor Operation lamps flash slowly, which customers often mistake for a fault.[37]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II strainers and valves are checked by touch or contact thermometer: a strainer or an open EEV/solenoid valve should show no temperature difference between inlet and outlet; a clear difference across a strainer means it is blocked and must be replaced.[38]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II solenoid valve SV2 (AJ*A36/45/54LALH, CN107) is closed in normal running and opens only in high or low pressure protection. Its coil is about 1495 Ω ±7 % and takes rated AC from the PCB. If it stays open when the plug is pulled, it is held by dirt or was deformed by brazing heat: replace the valve; if it closes, the PCB drives it wrongly.[39]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): When a J-II system does not cool, also check the pipe length setting entered on the outdoor unit (push buttons MODE/EXIT, SELECT, ENTER) against the installed piping, and that the indoor fan runs on high, the filter and coil are clean and the service valves are open.[40]
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Terms in this article

SuperheatSubcoolingSaturation temperature

Sources

  1. Danfoss Ref Tools – Troubleshooter: compressor
  2. Danfoss – User guide superheat controller EKE 1A/1B/1C/1D, BC398828796060en-000103 (2024.08)
  3. Danfoss – Installation guide Colibri electric expansion valves ETS 12C–100C, AN22038644422502-000501 (2017.09)
  4. Danfoss – Installation guide electric expansion valve ETS 12.5/ETS 25, AN24818649707403-000303 (2025.12)
  5. Castel – Solenoid Valves Handbook, VS-ED 01/2017-ENG (2017)
  6. Dixell – XW60K operating manual + Dixell – XR06CX operating manual + Pego – ECP202 EXPERT manual (rev. 02-25) and ECP200 EXPERT PULSE (rel. 01-24), p. 4 (odC, rrd) | p. 2 (rd) | p. 24 (Ed)
  7. AHT – Operating instructions plug-in cabinets Athen … Sydney (32DE-EN-FR-ES-PT), PDF p. 36 (intended use, climate class)
  8. Danfoss – Heat Pump Troubleshooting Guide (air and ground source heat pumps), VMIFB202, PDF p. 8 and p. 14
  9. Danfoss – Heat Pump Troubleshooting Guide (air and ground source heat pumps), VMIFB202 + NIBE – S1255 installer manual IHB 631735-2, PDF p. 28 | PDF p. 66 (comfort disturbances)
  10. Heatcraft – Unit Coolers Installation and Operations Manual H-IM-UC (June 2026), p. 15, table 3
  11. Hussmann – Transcritical CO2 Rack Installation & Operation, PN 3182569_G, p. 32, 71–72
  12. Hillphoenix – Advansor CO2 Booster – Installation, Start-Up and Operation manual, p. A-17
  13. Toshiba – Service Manual SVM-24053, RAV-GM1401AT8P-E1 Digital Inverter outdoor unit (R32), Oct 2024, 12. Control of compressor case thermo, p. 45; check code H04
  14. Hillphoenix – PF-48-R/PF-51-R/PF-54-R/PF-72-R Multi-deck Installation & Operations Manual rv6; Hillphoenix – JNRZHSA/JNRBHSA (R-290) I&O Manual 9.0 P119505H, PF case data, pdf p. 7; JNRZHSA case data, pdf p. 37
  15. Hillphoenix – JNRZHSA/JNRBHSA (R-290) I&O Manual 9.0 P119505H, Leak checking and repair / Charging, pdf p. 32
  16. ActronAir – ESP Platinum Plus/Ultima Service Manual 9590-2001 v1.03, Sec. 13 Trouble Shooting Guide, Sec. 13 p. 1
  17. Goodman/Daikin – DX20VC condensing unit, Installation & Service Reference IOD-4011L, p. 19, analysis guide
  18. Carel – EVD evolution user manual +0300005EN rel. 4.2 (24.01.2024), p. 50, Tab. 10.a
  19. Goodman/Daikin – DX20VC condensing unit, Installation & Service Reference IOD-4011L + Temperzone – Service Training Manual, Issue 08, January 2023, Goodman p. 19; Temperzone p. 248, §4.8
  20. Hillphoenix – CNZLA/CNEZLA (R-290) single-deck merchandiser I&O manual rv4 + Hussmann – MDN open vertical merchandiser I&O, P/N 9FQ-6-P110-075-00_B, Hillphoenix CNZLA p. 30 (troubleshooting) and installation section; Hussmann MDN pp. 2-8 – 2-9
  21. HTPG – Troubleshooting Guide EcoNet enabled unit coolers, HTPG0425_TSG (Jan 2025) + Danfoss – User Guide Superheat controller EKE 100 (PV04), BC457625021427en-000201 + Carel – EVD mini manual +0300036EN rel. 1.2 (2018-04-23), HTPG p. 10; EKE 100 p. 33 (A26); EVD mini p. 33
  22. Actron Air – Troubleshooting Guide Advance / Aires / VCC / Genesis, Doc. 9590-2015-03 Ver. 12 + Toshiba – Service manual multi-split RAS-5M34G3AVG-E, SVM-23012 (2023), Actron p. 4; Toshiba SVM-23012 p. 109
  23. Hussmann – Installation & Operation manual FS-OB, 3214313, pp. 35-36
  24. Hussmann – Installation & Operation manual FS-OB, 3214313 + Hillphoenix – Installation & operations manual OB-S, rv2, FS-OB p. 35; OB-S p. 8
  25. Hoshizaki – Service manual Steelheart UR reach-in + Beverage-Air – Installation and operating instructions UCF freezers, 809-174A, Steelheart p. 39; UCF p. 20
  26. Actron Air – Installation and Commissioning Guide Split Tri-Capacity 500–700, Doc. 0525-098 Ver. 06, pp. 78-79 (29.01)
  27. Actron Air – Advance Split Ducted Troubleshooting Guide 9590-2015-03 Ver. 3 + Actron Air – VCC and Genesis 70–100 kW Troubleshooting Guide 9590-2026-03 Ver. 3, 01 General troubleshooting and 05.01 status codes, p. 4 and 17; 01 General troubleshooting, p. 4
  28. Toshiba – Service Manual Split Type, File No. SVM-18049-5, 11-10 How to judge outdoor fan motor, p. 110
  29. Actron Air – Serene DS Series Troubleshooting, Doc. 9590-2037-03 Ver. 1, TS06 Refrigerant leakage detection
  30. Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), p. 3, A General Guide (Pipework, Outdoor Unit) (undated; copy archived 2026-11-02)
  31. Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models, Wall Mounted table: indoor fan lock and speed errors (6 + 2 and 6 + 3 flashes)
  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, 7. Maintenance, Refrigerant Re-Charge and operating charge table, p. 13–14
  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, 3.3 Compressor Capacity Control, p. 6
  34. 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, 3.2 Heating Operation, p. 5; 3.3 Compressor Capacity Control, p. 6; 3.4 Control of Electronic Expansion Valve, p. 7
  35. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 16, p. 04-105 (undated; copy archived 2026-11-03)
  36. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 4–6, p. 04-15–04-17 (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, 2-5-1 Oil recovery operation, p. 02-09; 4-1 Normal operation, p. 04-01–04-02 (undated; copy archived 2026-11-03)
  38. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 52, p. 04-61; service parts information 8–11, p. 04-92–04-95 (undated; copy archived 2026-11-03)
  39. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 11, p. 04-95; 2-6-1 Protective function list, p. 02-11–02-12 (undated; copy archived 2026-11-03)
  40. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 52, p. 04-61 (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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