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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

Compressor will not start

A compressor that will not start: alarms and safety devices, no demand, power supply faults and a seized or failed compressor.

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

Work with the power isolated and follow the electrical safety rules. Start with the controls and alarms: most stops are caused by a safety device, not by a failed compressor. Manufacturers' troubleshooting charts also start at that end [1].

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 voltage step by step: the supply, after the fuses, the control voltage, through the safety chain (pressure switches, motor protection, flow switch), to the contactor coil and out to the compressor. Where the voltage disappears is where the fault is. Listen too: if the compressor hums without starting, it has voltage but does not get going.

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

Flowchart

Compressor will not start1. Is there an active alarm or a trippedsafety device (high or low pressure,motor protection, flow switch)?A · Likely causeA safety device is preventing start.YesNo2. Is the controller calling for operation(demand present, correct mode, delaysexpired)?B · Likely causeNo demand, or a delay or externalinterlock is active.NoYes3. Does the compressor get the correctvoltage on all phases when the contactorpulls in?C · Likely causePower supply fault: fuse, contactor,phase monitor, inverter or looseconnection.NoYesD · Likely causeThe compressor is mechanically seized orhas a winding fault, and hums or trips themotor protection.Compressor will not start1. Is there an active alarm or a tripped safety device(high or low pressure, motor protection, flowswitch)?A · Likely causeA safety device is preventing start.YesNo2. Is the controller calling for operation (demandpresent, correct mode, delays expired)?B · Likely causeNo demand, or a delay or external interlock isactive.NoYes3. Does the compressor get the correct voltage on allphases when the contactor pulls in?C · Likely causePower supply fault: fuse, contactor, phasemonitor, inverter or loose connection.NoYesD · Likely causeThe compressor is mechanically seized or has a windingfault, and hums or trips the motor protection.

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

    A safety device is preventing start.

    Remedy: Read the alarm log and find the cause. Reset only when the cause has been fixed.

    🛠 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.

  2. 2
    Flowchart B

    No demand, or a delay or external interlock is active.

    Remedy: Check setpoints, operating mode, timers and inputs in the controller.

    🛠 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.

  3. 3
    Flowchart C

    Power supply fault: fuse, contactor, phase monitor, inverter or loose connection.

    Remedy: Measure the voltage before and after the contactor and inverter. Check phase sequence and fuses.

    🛠 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.

    Measure voltage and current under load

    Low voltage, phase imbalance or high current make motors overheat and trip. Measure at standstill, during start and in operation and compare with the nameplate. Live measurements in an open panel are for qualified personnel only.

    1. Prepare for live measurement. Use a CAT III/IV multimeter with insulated probes and a true-RMS clamp meter with inrush function. Wear safety glasses, no loose clothing, and keep hands clear of fans and belts that can start automatically.

      ✓ Check: Meter and leads are CAT III or CAT IV rated and the meter works on a known source.

    2. Measure the supply at standstill. Measure L1–L2, L2–L3 and L3–L1 (and phase to neutral where used) at the incoming terminals with the compressor stopped.

      ✓ Check: All voltages are within the range on the nameplate or data sheet (many compressors allow ±10 % of rated voltage).

    3. Measure voltage during start and in operation. Measure the same phases at the compressor contactor while the compressor starts and again after a few minutes of running. A large drop during start points to a weak supply, undersized cables or a loose connection.

      ✓ Check: The voltage stays within the permitted range also during start and in operation.

    4. Calculate the voltage imbalance. Imbalance (%) = largest deviation from the average of the three phase-to-phase voltages ÷ average × 100.

      ✓ Check: The imbalance is within the compressor manufacturer's limit (often about 2 %).

    5. Measure the current on each phase. Clamp each phase conductor separately: the inrush current at start and the running current after the system has stabilised.

      ✓ Check: The running current does not exceed the rated load amps (RLA) on the nameplate and is about equal on all three phases.

    6. Record and act. Write voltages, imbalance and currents in the service report. High voltage drop: tighten and check connections under isolation. Imbalance or high current: trace the supply or the motor before restarting repeatedly.

      ✓ Check: Readings are documented and any fault is traced to supply, connection or motor.

    Check and correct phase sequence

    Scroll and screw compressors and many fans must run in the right direction. A scroll compressor running backwards is noisy, does not pump and is damaged quickly.

    1. Measure the phase sequence. With the unit stopped, connect a phase rotation meter to L1, L2 and L3 at the unit's supply terminals (live measurement: CAT III/IV, insulated probes). If the unit has a phase monitoring relay, read its indicator too.

      ✓ Check: The meter shows right-hand (L1-L2-L3) rotation, or you know that it is reversed.

    2. Decide where to swap. If the whole unit is reversed, swap at the unit's supply terminals after the isolator. If only one motor runs backwards, swap at that motor. For a motor on an inverter, the input sequence does not matter; change direction with a parameter or at the drive output.

      ✓ Check: You know which two conductors to swap and where.

    3. Isolate, lock off and prove dead. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all phases and to earth.

      ✓ Check: The tester shows no voltage on any phase.

    4. Swap two phases. Swap two phase conductors (for example L1 and L2). Never move the earth or neutral conductor. Tighten the terminals to the specified torque.

      ✓ Check: Two phases are swapped and the terminals are tight.

    5. Check the sequence again. Remove your lock, switch on and measure the phase sequence again; check that the phase monitoring relay is satisfied.

      ✓ Check: The meter shows right-hand rotation.

    6. Start and confirm direction. Start the compressor and watch the gauges. Stop at once if it sounds wrong or the pressures do not separate.

      ✓ Check: Within seconds suction pressure falls and discharge pressure rises, the current is normal and the compressor runs quietly.

    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.

  4. 4

    The internal motor protector has opened because the compressor is overheated; it closes again only when the motor has cooled, which can take hours.

    Remedy: Feel whether the compressor shell is hot. With the power isolated, measure the windings: open between common and the other terminals but normal between run and start means the protector is open. Let it cool and find the cause of the overheating (see high discharge temperature).

    🛠 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.

    Safe isolation and lock-off

    Before you open a panel or touch electrical parts, isolate every supply, lock it off and prove that the circuit is dead. Electrical work is for qualified personnel only.

    1. Identify all supplies. Use the wiring diagram to find every supply: main feed, separate control supply, heaters, alarm and UPS supplies, and inverters that can feed back.

      ✓ Check: You know every isolation point for the equipment you will work on.

    2. Switch off, lock and tag. Switch each isolator to OFF, fit your own padlock and attach a tag with your name and date. Keep the key on you.

      ✓ Check: Every isolator is locked in OFF and cannot be switched on by someone else.

    3. Prove the tester. Check the two-pole voltage tester (EN 61243-3) on a proving unit or a known live source.

      ✓ Check: The tester shows the voltage of the known source.

    4. Prove dead. Measure phase to phase, each phase to neutral and each phase to earth at the point of work, on both line and load side.

      ✓ Check: The tester shows 0 V on every combination.

    5. Prove the tester again. Check the tester on the proving unit once more, to be sure it did not fail during the measurement.

      ✓ Check: The tester again shows the voltage of the known source.

    6. Deal with stored energy. On an inverter, wait the discharge time printed on the drive and measure the DC bus. Discharge start and run capacitors through a resistor of about 20 kΩ, 5 W with insulated leads, never with a screwdriver.

      ✓ Check: 0 V DC on the DC bus and across every capacitor.

    Measure compressor windings and insulation

    Winding resistance shows whether a motor winding is open, shorted or unbalanced, and insulation resistance shows whether it leaks to earth. Together they tell you whether the compressor motor is faulty before you replace it.

    1. Isolate, lock off and prove dead. Switch off and lock the isolator, put up a tag and prove dead with a two-pole tester between all phases and between each phase and earth at the compressor terminals.

      ✓ Check: The tester shows 0 V on every combination.

    2. Discharge capacitors and disconnect. Discharge any start or run capacitor through a resistor (about 20 kΩ, 5 W, insulated leads), never with a screwdriver. Photograph the wiring, then remove all cables from the compressor terminals. Disconnect any inverter and electronic protection module.

      ✓ Check: The meter shows 0 V DC across the capacitor terminals and the compressor terminals are free.

    3. Let a hot compressor cool. If the compressor is hot, an internal protector may be open. Measure the shell temperature; let it cool (it can take an hour or more) before judging the windings.

      ✓ Check: The shell is close to room temperature, or you have noted its temperature.

    4. Measure the winding resistance. Use the low-ohm range. Single-phase: measure C–R, C–S and R–S; C–R plus C–S should equal R–S. Three-phase: measure between the three terminal pairs; the values should be nearly equal. Compare with the compressor data sheet at the measured temperature. Open between C and both R and S while R–S reads normal points to an open internal protector.

      ✓ Check: The readings agree with the data sheet and with each other; OL means an open winding or protector.

    5. Measure the insulation resistance. Never megger under vacuum or with electronics connected. Connect the insulation tester (500 V DC, or the voltage the manufacturer gives) between each terminal and a clean earth point on the compressor body. Liquid refrigerant in the shell lowers the reading; run the crankcase heater and measure again before condemning the motor.

      ✓ Check: The reading is above the minimum in the compressor manufacturer's data; a reading near zero means the winding is earthed.

    6. Reconnect or replace. If the motor is good, reconnect from your photo, refit the cover and test run while measuring the current. If it is faulty, plan a compressor replacement and test the oil for acid.

      ✓ Check: The compressor runs at nameplate current, or the fault is recorded and a replacement planned.

  5. 5

    A loose cable, terminal or switch in the safety chain.

    Remedy: Tighten the terminals and measure along the chain until the voltage disappears.

    🛠 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.

    Trace the control and safety chain

    When a compressor or fan does not get a start signal, follow the control voltage through the safety chain in the wiring diagram to the point where it disappears. Live measurements in an open panel are for qualified personnel only.

    1. Get the wiring diagram. Find the diagram for the unit and mark the path from the control supply through fuses, safety devices (HP/LP switches, motor protection, thermal protection) and controller outputs to the contactor coil. Use a CAT III/IV meter with insulated probes.

      ✓ Check: The diagram matches the unit's model and serial plate, and you know the order of the devices.

    2. Check the control supply. Measure the voltage at the secondary of the control transformer or power supply and check the control fuses.

      ✓ Check: The rated control voltage (e.g. 24 V AC or 230 V AC) is present and the fuses are intact.

    3. Follow the chain device by device. Measure from each terminal in the chain to neutral or control common, in the order of the diagram. Never bridge a safety switch to make the system run.

      ✓ Check: You find the point where voltage is present on the input but 0 V on the output: that device is open.

    4. Find out why the device is open. Read the device: is the HP switch tripped and does it have a manual reset, is the motor protection tripped, is a door or flow switch open? Check the cause (pressure, current, temperature) before resetting.

      ✓ Check: The cause of the trip is known, not only the device.

    5. Check the controller output and relays. With a call for operation active, measure the output at the controller relay terminal. A relay that is energised but has no voltage on its output, or that stays closed without a call, is faulty.

      ✓ Check: The output follows the call: voltage when the controller calls and none when it does not.

    6. Repair and test. Isolate and lock off before replacing a part or tightening terminals. Restore power, reset and check that the chain closes and the contactor pulls in.

      ✓ Check: The system starts on call and stops when each safety device opens.

  6. 6

    A failed contactor coil or burnt contactor contacts.

    Remedy: Measure the voltage on the coil and across the contacts. Replace the contactor.

    🛠 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 contactor

    Worn, welded or loosely connected contactor contacts cause single-phasing, overheating and motor damage. Check the contacts, the coil and the voltage drop, and replace a worn contactor. Electrical work is for qualified personnel only.

    1. Isolate and lock off. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all line and load terminals, also the coil supply, which can come from a separate control circuit.

      ✓ Check: 0 V on all poles, the coil terminals and to earth.

    2. Inspect the contacts and the mechanism. Remove the arc cover if the design allows. Look for heavily pitted, burnt or welded contacts, and discoloured terminals or melted plastic. Press the armature by hand (power off) and check that it moves freely and springs back.

      ✓ Check: Contacts are without heavy pitting or welds, and the armature moves freely.

    3. Check the coil. Disconnect one coil lead and measure the coil resistance. Compare with the manufacturer's data or a new contactor of the same type.

      ✓ Check: The coil is neither open (OL) nor near 0 Ω.

    4. Check the coil voltage and pull-in. Reconnect, remove your lock, restore power and use the CAT III/IV meter to measure the voltage across the coil when the controller calls for operation.

      ✓ Check: The coil voltage matches the rating printed on the coil, and the contactor pulls in firmly without humming or chattering.

    5. Measure the voltage drop across each pole. With the motor running, measure from line to load terminal across each closed pole.

      ✓ Check: The drop is close to 0 V and about equal on all poles; a pole with clearly higher drop has worn contacts.

    6. Tighten or replace. Isolate and lock off again. Tighten all terminals to the manufacturer's torque. Replace a worn contactor with one of the same coil voltage and at least the same motor rating (AC-3), then restart and measure the current on each phase.

      ✓ Check: Terminals are tight, and the motor current is about equal on all phases after restart.

  7. 7

    Start capacitor or start relay (single phase) has failed.

    Remedy: Measure the capacitance and check the relay. Replace what has failed.

    🛠 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 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.

  8. 8
    Flowchart D

    The compressor is mechanically seized or has a winding fault, and hums or trips the motor protection.

    Remedy: Isolate and lock off the power, then measure winding and insulation resistance and check the starting equipment. 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.

    Safe isolation and lock-off

    Before you open a panel or touch electrical parts, isolate every supply, lock it off and prove that the circuit is dead. Electrical work is for qualified personnel only.

    1. Identify all supplies. Use the wiring diagram to find every supply: main feed, separate control supply, heaters, alarm and UPS supplies, and inverters that can feed back.

      ✓ Check: You know every isolation point for the equipment you will work on.

    2. Switch off, lock and tag. Switch each isolator to OFF, fit your own padlock and attach a tag with your name and date. Keep the key on you.

      ✓ Check: Every isolator is locked in OFF and cannot be switched on by someone else.

    3. Prove the tester. Check the two-pole voltage tester (EN 61243-3) on a proving unit or a known live source.

      ✓ Check: The tester shows the voltage of the known source.

    4. Prove dead. Measure phase to phase, each phase to neutral and each phase to earth at the point of work, on both line and load side.

      ✓ Check: The tester shows 0 V on every combination.

    5. Prove the tester again. Check the tester on the proving unit once more, to be sure it did not fail during the measurement.

      ✓ Check: The tester again shows the voltage of the known source.

    6. Deal with stored energy. On an inverter, wait the discharge time printed on the drive and measure the DC bus. Discharge start and run capacitors through a resistor of about 20 kΩ, 5 W with insulated leads, never with a screwdriver.

      ✓ Check: 0 V DC on the DC bus and across every capacitor.

    Measure compressor windings and insulation

    Winding resistance shows whether a motor winding is open, shorted or unbalanced, and insulation resistance shows whether it leaks to earth. Together they tell you whether the compressor motor is faulty before you replace it.

    1. Isolate, lock off and prove dead. Switch off and lock the isolator, put up a tag and prove dead with a two-pole tester between all phases and between each phase and earth at the compressor terminals.

      ✓ Check: The tester shows 0 V on every combination.

    2. Discharge capacitors and disconnect. Discharge any start or run capacitor through a resistor (about 20 kΩ, 5 W, insulated leads), never with a screwdriver. Photograph the wiring, then remove all cables from the compressor terminals. Disconnect any inverter and electronic protection module.

      ✓ Check: The meter shows 0 V DC across the capacitor terminals and the compressor terminals are free.

    3. Let a hot compressor cool. If the compressor is hot, an internal protector may be open. Measure the shell temperature; let it cool (it can take an hour or more) before judging the windings.

      ✓ Check: The shell is close to room temperature, or you have noted its temperature.

    4. Measure the winding resistance. Use the low-ohm range. Single-phase: measure C–R, C–S and R–S; C–R plus C–S should equal R–S. Three-phase: measure between the three terminal pairs; the values should be nearly equal. Compare with the compressor data sheet at the measured temperature. Open between C and both R and S while R–S reads normal points to an open internal protector.

      ✓ Check: The readings agree with the data sheet and with each other; OL means an open winding or protector.

    5. Measure the insulation resistance. Never megger under vacuum or with electronics connected. Connect the insulation tester (500 V DC, or the voltage the manufacturer gives) between each terminal and a clean earth point on the compressor body. Liquid refrigerant in the shell lowers the reading; run the crankcase heater and measure again before condemning the motor.

      ✓ Check: The reading is above the minimum in the compressor manufacturer's data; a reading near zero means the winding is earthed.

    6. Reconnect or replace. If the motor is good, reconnect from your photo, refit the cover and test run while measuring the current. If it is faulty, plan a compressor replacement and test the oil for acid.

      ✓ Check: The compressor runs at nameplate current, or the fault is recorded and a replacement planned.

    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.

Checks from the manufacturers’ manuals (49)

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

  • Danfoss AK-CC55 Compact: alarm a13 (compressor lock-out after too many trips) needs a manual reset: hold the alarm button 3 s in the alarm list, power-cycle the controller, or switch r12 off and on.[2]
  • Copeland CoreSense Diagnostics: a steady red TRIP LED means run demand but no compressor current; check the internal protector, fuses/breaker, an open HP switch, the wiring and the contactor.[3]
    red steady
  • Secop BD: 3 LED flashes (about 1/4 s each, repeated every 4 s) mean the motor could not start: rotor blocked or more than about 5 bar pressure difference; let the pressures equalise first.[4]
    red ●●● · 0.25 s on, 0.25 s off · pause 2.75 s
  • Tecumseh hermetic compressors: below 2 MΩ between any terminal pin and the copper suction line (megger or Hi-Pot, not an ohmmeter) is a ground fault; do not energise again, replace the compressor.[5]
  • Tecumseh hermetic compressors: single-phase C–S plus C–R should roughly equal S–R; on three-phase, all three leg-to-leg readings should be above zero and within about 10 % of each other.[5]
  • Copeland Electronic Unit Controller: HP flashing at start-up means the jumper harness is not connected or the HP switch sees pressure above cut-out; when replacing a -00 controller, fit the new blue-wire jumper.[6]
  • Carlyle 06D/06E/06CC that will not start: check main switch, fuses and wiring, then the manually reset overcurrent protection and oil safety switch, and finally the motor for a burnout.[7]
  • Mitsubishi Electric SUZ-KA outdoor units: Inverter split unit: disconnect the compressor from the inverter and run a test; the three output voltages (about 50–130 V AC, analog meter) must be balanced, otherwise the inverter board is faulty. Then reconnect and time the stop on overcurrent: about 1 s points to the power module, about 2 s to a winding short, within about 10 s to a locked compressor or poor contact, within 10 minutes to the refrigerant circuit (for example a closed valve).[8]
  • Tecumseh AG/TAG; Danfoss MT/MTZ: The compressor is full of liquid (hydraulic lock) after overcharging, for example because the charging cylinder was left connected with a slightly leaking valve; the motor cannot turn and behaves like a locked rotor.[9]
  • Danfoss AK-RC 113 panel with display off: the transformer secondary fuse (F250 mA) blows if the load exceeds about 0.25 A, for example when extra devices are connected to the Kriwan supply. At first start, jumper the inputs of devices that are not fitted (pressure switches, Kriwan) and set the pump-down/thermostat mode bridge.[10]
  • Danfoss CDS303/803 drive (VZH/VLZ scroll): read the alarm log (last 10 alarms) before changing parts. Motor phase missing (30–32) usually means the motor cable; mains failure (36) is often a voltage dip, for example another unit on the same supply starting; safe stop (68) means the HP switch opened, so reset it and then the drive. Over current, start failure or speed limit (13, 18, 49) point to the compressor.[11]
  • NIBE S1255: Heat pump that does not start the compressor: wait at least 30 minutes, since a minimum time between starts may be running, and check that the temperatures are within the working range, which also blocks the compressor.[12]
  • Temperzone UC7: three trips of the same protection while the compressor is called for put the unit in lock-out, which only removing and restoring mains power clears; afterwards the old fault is shown for 45 s at each power-up until one normal compressor cycle has run.[13]
  • Secop BD35F/BD50F electronic units: 12/24 V DC compressor: connect the cable straight to the battery poles with the cross-section the length requires, a fuse close to the battery (15 A at 12 V, 7.5 A at 24 V) and no extra joints; voltage drop trips the battery protection (one LED flash; standard cut-out about 10.4 V and cut-in 11.7 V on 12 V, 22.8/24.2 V on 24 V).[14]
  • Danfoss AK-PC 782B: In a pack controller the 'active at' setting (open or closed) of a compressor safety input does not match how the safety is wired, so the controller sees a tripped safety and will not start the compressor; toggle each input and check its status.[15]
  • Tecumseh hermetic compressors: Single-phase starting parts: a start capacitor should measure its rated value −0/+20 % and a run capacitor within ±10 %; discharge them through about 20 kΩ before testing. A current relay must be tested upright (L–S open, L–M closed), and a PTC starter needs about 3 minutes to cool before a continuity test means anything.[16]
  • Tecumseh hermetic compressors: Repeated start capacitor or relay failures usually have an outside cause: supply below about 90 % of rated voltage (long start time), a relay of the wrong type or mounted at the wrong angle or loosely, frequent short cycling, or a run capacitor that fails from supply above about 110 %.[17]
  • Temperzone UC8: On some unit controllers fan or compressor overload contacts are wired in series with the compressor run signal, so a tripped overload simply stops the compressor without any fault on the display. Measure through the run signal chain before suspecting the controller.[18]
  • Hussmann transcritical CO2 racks; Hillphoenix Advansor: CO2 rack that stops all compressors: a phase-loss monitor contact, high discharge pressure or all MT compressors locked out puts the rack in an emergency stop with HPV and flash gas valve closed and case EEVs shut. Without a 1–3 s delay on the phase monitor input, short voltage dips or a generator changeover trip it.[19]
  • Danfoss VLT Refrigeration Drive FC 103: Compressor on a frequency drive does not start but the display works: check that [Off] has not been pressed on the keypad, that a start signal is on the start input (FC 103: terminal 18), and that the coast input has 24 V (FC 103: terminal 27, or programmed to no function); a service switch between drive and motor also stops it.[20]
  • Bitzer SE-B/SE-E/SE-i compressor protection devices: A compressor protection relay in the safety chain can fail to make reliable contact if the circuit current is too small: Bitzer's AC relay versions need at least about 20 mA, while the 24 V DC versions with gold contacts are ruined by more than about 10 mA. A PLC input on an AC contact, or a contactor coil on a DC contact, gives intermittent no-starts.[21]
  • Heatcraft Beacon II refrigeration control: Low 24 V control voltage: evaporator control transformers are often shipped on the 240 V tap, and on a 208 V supply they must be moved to the 208 V tap. Too low control voltage makes the board reset or the contactor drop out right after the start delay (Beacon II: 888 on the display below about 18 V).[22]
  • Secop BD compressors with electronic units 101N0212/0340/0390/0420/0510/0650: 12/24 V DC compressors (boats, vehicles, solar): too thin or too long battery cables drop the voltage at start, so the controller's battery protection stops the compressor although the battery itself is fine. Measure at the controller terminals during start; Secop allows only about 2.5 m of 2.5 mm² at 12 V (5 m at 24 V), and the default cut-out is about 10.4 V / 22.8 V after 3 s.[23]
  • Toshiba RAV-GM digital inverter split systems: On split units the compressor is blocked by an indoor-unit drain overflow (float switch) alarm, by the 3-minute restart delay, and for up to about 5 minutes after first power-on or a change of indoor address while auto-addressing runs.[24]
  • Vaillant aroSTOR VWL B/BM 200/6 and 260/6: Domestic hot water heat pump with no fault code but a stopped compressor: the manual-reset HP switch (opens at 30 bar, resets only below 24 bar) has tripped; a dark display means the 95 °C thermal cut-out (about 32 bar) has tripped. Find the cause (blocked circuit) before pressing the reset button under the cover.[25]
  • Temperzone units with Carel Power+ inverter: An inverter compressor stalls at start and the drive gives up after several attempts because liquid refrigerant has migrated into the compression chamber during the off period. Check the sump heater; on Temperzone UC8 units it runs for 4 hours after each power-up, so power the unit 4 hours before commissioning.[26]
  • Scotsman Prodigy ice machines: Single-phase compressor with a PTCR start device: at room temperature (about 10–38 °C) the PTCR should read about 25–50 Ω and be open to ground. If the start current does not drop at once after start, replace the PTCR; high running current with the compressor running points to an open run capacitor. Replace PTCR and run capacitor together with a new compressor.[27]
  • Hussmann Protocol: Some rack controllers will not start any compressor while the suction pressure reads below about 0.14 bar (2 psig), as vacuum protection; check the transducer reading before suspecting the starter.[28]
  • Scotsman SCE275 ice machine: Single-phase compressors with a PTC start device: let the PTC cool for at least 5 minutes before testing, since a hot PTC blocks a quick restart. Scotsman replaces it if the cold resistance is outside 22-40 ohm.[29]
  • Actron Air Aires/Advance with VSD: Inverter compressor that fails to start or trips on over-current: the compressor leads U-V-W are swapped or loose, the pressure difference at start is too high (a closed valve or a restriction), or liquid or oil has collected in the compressor.[30]
  • Copeland ZF/ZB K5 scroll: After a PTC trip, the INT69 module on Copeland K5 scrolls waits about 30 minutes before it releases, even when the motor has cooled; check the module before condemning the compressor.[31]
  • Copeland scroll compressors: A low insulation reading (megger or hipot) can come from liquid refrigerant in the shell, especially with the motor at the bottom. Run the compressor briefly or warm it, then measure again. Never megger a compressor under vacuum.[31]
  • Hussmann CO2 racks: Rack shuts down on its phase loss monitor at short voltage dips or when switching to generator power, because the input has no delay. A 1–3 s delay on the input in the rack controller avoids false trips.[32]
  • Fujitsu compressor position or start error (1A): check that both service valves are fully open, measure the compressor winding resistances and look for loose compressor leads before replacing the compressor or the inverter PCB.[33]
  • Fujitsu split air conditioners and heat pumps: A Fujitsu split that seems not to start may simply be in timer or test mode; on older models the timer runs only once unless repeat is set, and the small heat pump models 7–14 have no automatic heating/cooling changeover.[34]
  • Fujitsu: powering an external condensate pump from the indoor unit is a common cause of communication and other errors; give the pump its own supply.[35]
  • Fujitsu communication errors (01, 13): check the cable from the isolator to the outdoor unit and on to the indoor unit, and the voltage at all three points. Fujitsu advises against wire nuts and splices in the interconnecting cable; use crimped ring terminals. The signal wire carries digital pulses that cannot be read with a normal multimeter.[36]
  • Fujitsu wired remote error (00): measure about 12 V DC at the remote connector on the indoor PCB; if the voltage is there the remote itself is faulty. The remote cable should be screened and kept away from mains cables.[37]
  • Fujitsu split air conditioners and heat pumps: On older Fujitsu splits with an LCD wired remote, terminal 3 of the interconnection is the live and terminal 1 must never get mains voltage; check the wiring against the model before powering up after a replacement.[38]
  • Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): The Hitachi RUA-NP scroll compressors may only turn one way, so the unit has a reverse and single-phase protection relay: with the phases in the wrong order or one phase missing the compressor will not start and SEG1 shows 05. Check with a phase-sequence meter and swap two incoming wires at the main supply terminals, never inside the unit. The three-phase condenser fan must also be checked for rotation at first start.[39]
  • Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): A start delay is normal: after COOL is selected the evaporator fan runs at once, but the compressor and condenser fan start only after the 3-minute anti-short-cycle timer on the PCB. On the two-compressor RUA-NP9/NP10 the second compressor follows after 3.5 minutes, and the start order of compressor 1 and 2 changes with every first-stage thermostat call.[40]
  • Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): If only one compressor of a two-compressor RUA-NP runs, check DIP switch DSW2: it has settings that force compressor 1 or compressor 2 to stay off. DIP switch changes are read only at power-up, so a setting changed with the power on has no effect until the power is switched off and on; code 31 means the capacity switch DSW1 is not set.[41]
  • Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: A VRV indoor fan that stays off or at very low speed at the start of heating is normally hot start: after thermostat-on, defrost or oil return the fan waits until 3 minutes have passed, the gas pipe thermistor reads above about 34 °C (93 °F) or the condensing temperature is above about 52 °C (126 °F), so that no cold air is blown into the room.[42]
  • NTF SM flat flake ice makers (SM500–SM11000): NTF SM flakers: the SM4500 has a phase sequence relay that stops the machine from starting if the three phases are connected in the wrong order, and the SM1750 has a refrigerating circuit safety pressure switch (manual reset) that blocks start-up when the room is too hot.[43]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II inverter compressor: the winding resistance is about 0.24 Ω between each pair of terminals at 20 °C; 0 Ω or open means a faulty compressor. If windings and wiring are good, check the supply from the main PCB to the inverter PCB on CN126: 13.5–16.5 V DC between pins 1–2 and −12 to −8 V DC between pins 3–2; if both are present, the inverter PCB is faulty.[44]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): A J-II compressor that does not start is not always faulty: after a stop it waits 3 min for the pressure difference to equalise, and it only restarts when an indoor unit calls. Start-up error E.93.1 appears only after 60 restart attempts in each of 2 sets (at least about 100 min). Check the inverter PCB–compressor wiring, the IPM and the compressor for lock or shorted windings.[45]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II outdoor fan lock E.97.1/E.98.1: speed 100 rpm or less 20 s after the start command, 15 times; the compressor is stopped after every third fan stop. Turn the fan by hand, check the motor plug and windings, and the fan drive output on the main PCB (13.6–16.5 V DC). Fan overheat E.97.5/E.98.5 (220 rpm or less after 60 s, 3 times in 3 h) also points to a too high static pressure setting or blocked coil.[46]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II network design limits: exactly one terminal resistor per network segment, at most 500 m cable and 64 units per segment (an amplifier port counts as one), at most 8 signal amplifiers per system, no loops, the shield earthed at both ends and the network cable not bundled with power cables. No more than 64 indoor units may be on one refrigerant circuit (otherwise E.28.1). Breaking these rules gives communication errors and units missing from a scan.[47]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II lock-out errors (for example discharge or compressor temperature E.A1.1/E.A3.1, low pressure E.A5.1, current sensor, compressor start-up, trip, loss of synchronisation and fan lock) do not clear by themselves: after the cause is fixed, switch the outdoor unit off and on and then do an error reset (push button or function F3-40).[48]
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Terms in this article

High-pressure switchLow-pressure switch

Sources

  1. Danfoss Ref Tools – Troubleshooter: compressor
  2. Danfoss – User guide AK-CC55 Single Coil SW 2.4x, BC364229688105en-010401 (2026.08); User guide AK-CC55 Compact SW 2.3x, BC365212986197en-000302 (2026.08)
  3. Copeland (Emerson) – CoreSense Diagnostics for Copeland scroll air conditioning compressors, AE8-1379 R2 (February 2015)
  4. Secop – Instructions electronic unit for BD35F/50F/35K/80CN compressors, 101N0242/0680/0682, DES.I.101.X2.02 (June 2022)
  5. Tecumseh – Hermetic compressor service handbook, North America, REV 3/11 (2011)
  6. Copeland – Electronic Unit Controller, AE8-1376, section 9 (2013)
  7. Carlyle – Service Guide 06D, 06E and 06CC Semi-Hermetic Reciprocating Compressors, Lit No. 020-611 Rev. F (2/2026)
  8. Mitsubishi Electric – Technical & service manual OCH545 (SUZ-KA25–71VA4), PDF p. 29–30 (11-5 A–E)
  9. Tecumseh – AG/TAG detailed installation instructions (EU, EN) + Danfoss – Maneurop MT/MTZ/NTZ/VTZ instructions, AN13348643903904-000702, p. 10 (6.3) | p. 3 (8)
  10. Danfoss – Optyma control AK-RC 113 user guide, BC317523579312en-000101 (2020.03), p. 16 (7.2)
  11. Danfoss – CDS303/803 quick troubleshooting guide, AX404427814471en-000101 (2022.03) + Danfoss – VZH088-170 Gen3 application guide, AB300034185311en-000501, p. 2–5 | p. 38–39
  12. NIBE – S1255 installer manual IHB 631735-2, PDF p. 66 (printed p. 63)
  13. Temperzone – Troubleshooting Guide for UC7 Controller (1012-3), p. 11 (9. Lock-out)
  14. Secop – Electronic unit 101N0242/0680/0682 for BD35F/BD50F/BD35K/BD80CN, 12–24 V DC (06-2022), p. 1–2
  15. Danfoss – Testing and startup guide AK-PC 782B, BC479936513201en-000101 (2024.02), p. 6 (5. digital input verification)
  16. Tecumseh – Compressor Service Handbook (hermetic compressors), p. 45–55
  17. Tecumseh – Electrical Service Parts Guide Book, AF-110 Rev 1 (05/2017) | Tecumseh – Compressor Service Handbook (hermetic compressors), Tecumseh: p. 104–105 | Tecumseh: p. 51–55
  18. Temperzone – UC8 Troubleshooting Guide, issue 2 (27 July 2015), p. 14
  19. Hussmann – Transcritical CO2 Rack Installation & Operation, PN 3182569_G | Hillphoenix – Advansor CO2 Booster – Installation, Start-Up and Operation manual, Hussmann: p. 32 | Hillphoenix: p. A-15–A-16
  20. Danfoss – VLT Refrigeration Drive FC 103 Instruction Manual MG16E322 (2014-04-10), p. 46, table 7.5
  21. Bitzer – Technical information CT-120-3, compressor protection devices, p. 49–51 (CT-120-3)
  22. Heatcraft – Beacon II installation and operation manual H-IM-79G, p. 27, 33 (H-IM-79G)
  23. Secop – BD electronic units operating instructions DESS100F202 (07-2016), p. 5–6, 9–10 (DESS100F202, 07-2016)
  24. 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
  25. Vaillant – aroSTOR plus Installation and maintenance instructions 8000034704_01, 9.6 Resetting the safety devices, pdf p. 58; Appendix C, p. 63–64
  26. Temperzone – Service Training Manual, Issue 08, January 2023, pp. 283–285
  27. Scotsman – Prodigy Series modular cuber technical service manual (Jan 2015), p. 56, Test Procedures – Compressor
  28. Hussmann – Protocol distributed system I&O manual P/N 365841C, pp. 6-7 – 6-8 (PDF p. 77)
  29. Scotsman – Service manual SCE275 (January 2003), p. 29 (PTCR)
  30. Actron Air – Troubleshooting Guide Advance / Aires / VCC / Genesis, Doc. 9590-2015-03 Ver. 12, 05.03 pp. 28-29
  31. Copeland – Application Guidelines for ZF*K5E and ZB*K5E Copeland Scroll compressors, AE4-1383 R15 (2017), PTC Motor Protection, p. 11
  32. Hussmann – Transcritical CO2 Rack, Installation & Operation, PN 3182569_G, Liquid Injection, Table 7, p. 61
  33. Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models, Wall Mounted table: code 1A (5 + 5 flashes)
  34. Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), p. 3, A General Guide (Controller, Indoor Unit) (undated; copy archived 2026-11-02)
  35. Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), p. 2, note under Interconnecting Wiring (undated; copy archived 2026-11-02)
  36. Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models; Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), 2019: Wall Mounted table, codes 01 and 13; UK: p. 3, Wiring
  37. Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models; Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), 2019: Wall Mounted table, code 00 (Timer 8 flashes); UK: p. 3, Wiring
  38. Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), p. 3, Wiring (undated; copy archived 2026-11-02)
  39. 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, 3.1 Main Power Wiring Procedures, p. 4; 3.3.6 alarm code 05, p. 7
  40. 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, 6.3 Test Running, p. 10; Fig. 9-1/9-2 Operational Sequence, p. 11; 6.5 CCP timer, p. 12
  41. 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, 3.3.3–3.3.4 DIP switch settings, p. 5–6; 3.3.6 alarm code 31, p. 7
  42. 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, 4.4 Hot Start Control, p. 14
  43. 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), Electric circuit – Safety devices, page 144
  44. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 2, p. 04-85 (undated; copy archived 2026-11-03)
  45. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 34, p. 04-44; service parts information 1, p. 04-84 (undated; copy archived 2026-11-03)
  46. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 37 and 39, p. 04-47 and 04-49 (undated; copy archived 2026-11-03)
  47. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 46, 60 and 85, p. 04-56, 04-65 and 04-82 (undated; copy archived 2026-11-03)
  48. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble level of system, p. 04-09–04-10; 2-6-1 Protective function list, p. 02-11–02-12 (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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