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Troubleshooting · Applies to: Heat pump and air conditioning

Heat pump does not switch between heating and cooling

A four-way valve that does not switch: control signal, coil, too little pressure difference, and a seized or leaking valve.

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

The four-way valve is controlled by a small pilot valve with a coil. The pilot valve directs pressure that moves the main slide, so the valve can only switch while the compressor runs and there is a pressure difference. Check the control signal and the coil first and then the pressures, before condemning the valve itself [1]. Which position is de-energised differs between manufacturers: check the wiring diagram.

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

Check in the controller which mode is requested and measure the voltage on the coil. Feel the pipes: in heating mode the pipe to the indoor unit should be hot.

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

Flowchart

Heat pump does not switch betweenheating and cooling1. Does the coil get the correct voltagewhen the valve is supposed to switch?A · Likely causeThe control signal is missing: relay,control board, thermostat or cable.NoYes2. Is the coil intact (correct resistance)and magnetic, so a screwdriver is pulledtowards it?B · Likely causeThe coil is broken or incorrectly fitted.NoYes3. With the compressor running, is thedifference between high and low pressurenormal when the valve is supposed toswitch?C · Likely causeToo little pressure difference to movethe slide, often because of too littlerefrigerant.NoYesD · Likely causeThe valve is mechanically stuck or leaksinternally between the high and low side.Heat pump does not switch between heating andcooling1. Does the coil get the correct voltage when the valveis supposed to switch?A · Likely causeThe control signal is missing: relay, controlboard, thermostat or cable.NoYes2. Is the coil intact (correct resistance) andmagnetic, so a screwdriver is pulled towards it?B · Likely causeThe coil is broken or incorrectly fitted.NoYes3. With the compressor running, is the differencebetween high and low pressure normal when the valveis supposed to switch?C · Likely causeToo little pressure difference to move theslide, often because of too little refrigerant.NoYesD · Likely causeThe valve is mechanically stuck or leaks internallybetween the high and low side.

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

    The control signal is missing: relay, control board, thermostat or cable.

    Remedy: Follow the signal in the wiring diagram. Replace a faulty relay or board and tighten terminals.

    🛠 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.

  2. 2
    Flowchart B

    The coil is broken or incorrectly fitted.

    Remedy: Measure the resistance and replace the coil. Check that it is fully seated on the pilot valve.

    🛠 How do I do it? Step by step

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

    Check a four-way (reversing) valve

    A heat pump that will not change mode, or loses capacity, often has a reversing valve that does not get a signal, does not shift or leaks internally. Temperatures on the pipes tell you which.

    1. Find out the valve logic. Check in the wiring diagram or manual in which mode the coil is energised. This differs between makers: some energise in cooling, others in heating. Note the mode the controller asks for.

      ✓ Check: You know whether the coil should be on or off in the current mode.

    2. Check the coil and its signal. With a CAT III/IV meter, measure the voltage at the coil when the mode changes. Then isolate, lock off, prove dead, and measure the coil resistance. Check that the coil voltage marking matches the supply and that the coil sits fully on the valve stem.

      ✓ Check: The coil gets its rated voltage in the right mode and its resistance matches the data sheet; OL means a broken coil.

    3. Measure the pipe temperatures. With the compressor running, measure the four main pipes with clamp thermometers: discharge into the valve (hot), suction from the valve to the compressor (cold), and the two coil connections. Beware of the hot discharge pipe. The valve needs enough pressure difference to shift, so check with the compressor running.

      ✓ Check: The hot side follows the mode: to the outdoor coil in cooling, to the indoor coil in heating.

    4. Check the pilot tubes at switch-over. Change the mode and feel or measure the small pilot capillaries on the valve during the switch. Their temperatures should change as the pilot valve moves.

      ✓ Check: The pilot tube temperatures change at switch-over; if they do but the main pipes do not, the main slide is stuck.

    5. Look for internal leakage. Compare the temperature of the suction pipe between valve and compressor with the pipe coming from the coil that is working as evaporator. A clearly warmer pipe after the valve means hot gas is leaking across the slide.

      ✓ Check: Both pipes have nearly the same temperature; a large difference, together with low capacity, points to internal leakage.

    6. Try to free it, otherwise replace. Switch the mode several times with the compressor running and full pressure difference. If the valve still sticks or leaks, it must be replaced by F-gas certified personnel: recover the refrigerant, braze with nitrogen flowing and a heat sink or wet cloth on the valve body, pressure test, evacuate and weigh in the charge. With R32 or R290 follow the rules for flammable refrigerants (no ignition sources, ventilation, gas detector).

      ✓ Check: After repair the valve shifts in both directions and the pipe temperatures follow the mode.

  3. 3
    Flowchart C

    Too little pressure difference to move the slide, often because of too little refrigerant.

    Remedy: Leak test and correct the refrigerant charge. Switch with the compressor running and full pressure difference.

    🛠 How do I do it? Step by step

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

    Leak test a refrigeration system

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

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

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

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

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

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

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

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

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

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

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

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

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

    Check a four-way (reversing) valve

    A heat pump that will not change mode, or loses capacity, often has a reversing valve that does not get a signal, does not shift or leaks internally. Temperatures on the pipes tell you which.

    1. Find out the valve logic. Check in the wiring diagram or manual in which mode the coil is energised. This differs between makers: some energise in cooling, others in heating. Note the mode the controller asks for.

      ✓ Check: You know whether the coil should be on or off in the current mode.

    2. Check the coil and its signal. With a CAT III/IV meter, measure the voltage at the coil when the mode changes. Then isolate, lock off, prove dead, and measure the coil resistance. Check that the coil voltage marking matches the supply and that the coil sits fully on the valve stem.

      ✓ Check: The coil gets its rated voltage in the right mode and its resistance matches the data sheet; OL means a broken coil.

    3. Measure the pipe temperatures. With the compressor running, measure the four main pipes with clamp thermometers: discharge into the valve (hot), suction from the valve to the compressor (cold), and the two coil connections. Beware of the hot discharge pipe. The valve needs enough pressure difference to shift, so check with the compressor running.

      ✓ Check: The hot side follows the mode: to the outdoor coil in cooling, to the indoor coil in heating.

    4. Check the pilot tubes at switch-over. Change the mode and feel or measure the small pilot capillaries on the valve during the switch. Their temperatures should change as the pilot valve moves.

      ✓ Check: The pilot tube temperatures change at switch-over; if they do but the main pipes do not, the main slide is stuck.

    5. Look for internal leakage. Compare the temperature of the suction pipe between valve and compressor with the pipe coming from the coil that is working as evaporator. A clearly warmer pipe after the valve means hot gas is leaking across the slide.

      ✓ Check: Both pipes have nearly the same temperature; a large difference, together with low capacity, points to internal leakage.

    6. Try to free it, otherwise replace. Switch the mode several times with the compressor running and full pressure difference. If the valve still sticks or leaks, it must be replaced by F-gas certified personnel: recover the refrigerant, braze with nitrogen flowing and a heat sink or wet cloth on the valve body, pressure test, evacuate and weigh in the charge. With R32 or R290 follow the rules for flammable refrigerants (no ignition sources, ventilation, gas detector).

      ✓ Check: After repair the valve shifts in both directions and the pipe temperatures follow the mode.

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

  4. 4

    The pilot valve capillary tubes are blocked or damaged.

    Remedy: Feel the capillary tubes when switching: if they do not get warm or cold as expected they are blocked, and the valve is replaced.

    🛠 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 four-way (reversing) valve

    A heat pump that will not change mode, or loses capacity, often has a reversing valve that does not get a signal, does not shift or leaks internally. Temperatures on the pipes tell you which.

    1. Find out the valve logic. Check in the wiring diagram or manual in which mode the coil is energised. This differs between makers: some energise in cooling, others in heating. Note the mode the controller asks for.

      ✓ Check: You know whether the coil should be on or off in the current mode.

    2. Check the coil and its signal. With a CAT III/IV meter, measure the voltage at the coil when the mode changes. Then isolate, lock off, prove dead, and measure the coil resistance. Check that the coil voltage marking matches the supply and that the coil sits fully on the valve stem.

      ✓ Check: The coil gets its rated voltage in the right mode and its resistance matches the data sheet; OL means a broken coil.

    3. Measure the pipe temperatures. With the compressor running, measure the four main pipes with clamp thermometers: discharge into the valve (hot), suction from the valve to the compressor (cold), and the two coil connections. Beware of the hot discharge pipe. The valve needs enough pressure difference to shift, so check with the compressor running.

      ✓ Check: The hot side follows the mode: to the outdoor coil in cooling, to the indoor coil in heating.

    4. Check the pilot tubes at switch-over. Change the mode and feel or measure the small pilot capillaries on the valve during the switch. Their temperatures should change as the pilot valve moves.

      ✓ Check: The pilot tube temperatures change at switch-over; if they do but the main pipes do not, the main slide is stuck.

    5. Look for internal leakage. Compare the temperature of the suction pipe between valve and compressor with the pipe coming from the coil that is working as evaporator. A clearly warmer pipe after the valve means hot gas is leaking across the slide.

      ✓ Check: Both pipes have nearly the same temperature; a large difference, together with low capacity, points to internal leakage.

    6. Try to free it, otherwise replace. Switch the mode several times with the compressor running and full pressure difference. If the valve still sticks or leaks, it must be replaced by F-gas certified personnel: recover the refrigerant, braze with nitrogen flowing and a heat sink or wet cloth on the valve body, pressure test, evacuate and weigh in the charge. With R32 or R290 follow the rules for flammable refrigerants (no ignition sources, ventilation, gas detector).

      ✓ Check: After repair the valve shifts in both directions and the pipe temperatures follow the mode.

  5. 5
    Flowchart D

    The valve is mechanically stuck or leaks internally between the high and low side.

    Remedy: Replace the valve. Cool the valve body while brazing and braze with nitrogen flowing through the pipe.

    🛠 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 four-way (reversing) valve

    A heat pump that will not change mode, or loses capacity, often has a reversing valve that does not get a signal, does not shift or leaks internally. Temperatures on the pipes tell you which.

    1. Find out the valve logic. Check in the wiring diagram or manual in which mode the coil is energised. This differs between makers: some energise in cooling, others in heating. Note the mode the controller asks for.

      ✓ Check: You know whether the coil should be on or off in the current mode.

    2. Check the coil and its signal. With a CAT III/IV meter, measure the voltage at the coil when the mode changes. Then isolate, lock off, prove dead, and measure the coil resistance. Check that the coil voltage marking matches the supply and that the coil sits fully on the valve stem.

      ✓ Check: The coil gets its rated voltage in the right mode and its resistance matches the data sheet; OL means a broken coil.

    3. Measure the pipe temperatures. With the compressor running, measure the four main pipes with clamp thermometers: discharge into the valve (hot), suction from the valve to the compressor (cold), and the two coil connections. Beware of the hot discharge pipe. The valve needs enough pressure difference to shift, so check with the compressor running.

      ✓ Check: The hot side follows the mode: to the outdoor coil in cooling, to the indoor coil in heating.

    4. Check the pilot tubes at switch-over. Change the mode and feel or measure the small pilot capillaries on the valve during the switch. Their temperatures should change as the pilot valve moves.

      ✓ Check: The pilot tube temperatures change at switch-over; if they do but the main pipes do not, the main slide is stuck.

    5. Look for internal leakage. Compare the temperature of the suction pipe between valve and compressor with the pipe coming from the coil that is working as evaporator. A clearly warmer pipe after the valve means hot gas is leaking across the slide.

      ✓ Check: Both pipes have nearly the same temperature; a large difference, together with low capacity, points to internal leakage.

    6. Try to free it, otherwise replace. Switch the mode several times with the compressor running and full pressure difference. If the valve still sticks or leaks, it must be replaced by F-gas certified personnel: recover the refrigerant, braze with nitrogen flowing and a heat sink or wet cloth on the valve body, pressure test, evacuate and weigh in the charge. With R32 or R290 follow the rules for flammable refrigerants (no ignition sources, ventilation, gas detector).

      ✓ Check: After repair the valve shifts in both directions and the pipe temperatures follow the mode.

    Braze with a nitrogen purge

    Without nitrogen inside the pipe, brazing leaves oxide scale that blocks filters, expansion valves and oil strainers. A gentle flow of dry nitrogen keeps the inside clean. Certified personnel only.

    1. Recover and depressurise. Recover the refrigerant and prove the circuit is at 0 bar(g). Never braze a pressurised or refrigerant-filled pipe. With flammable refrigerants (A2L/A3), flush with nitrogen, ventilate and check with a gas detector that no refrigerant remains.

      ✓ Check: The gauge shows 0 bar(g) and the gas detector shows no refrigerant.

    2. Prepare for hot work. Obtain a hot work permit if required, remove flammable material, put a fire extinguisher at hand and shield nearby insulation and cables.

      ✓ Check: The permit is in place, the extinguisher is within reach and the area is clear.

    3. Prepare the joint. Cut the pipe square, deburr it, clean the surfaces and push the pipe fully into the fitting. Use the right filler: phosphor-copper without flux for copper to copper, silver solder with flux for copper to brass or steel.

      ✓ Check: The joint is clean and fully inserted and the filler matches the materials.

    4. Start a gentle nitrogen flow. Connect dry nitrogen through a regulator and a flow meter and let a gentle flow through the pipe, with an open outlet so the pressure cannot build up.

      ✓ Check: You can feel a gentle flow at the outlet and the gauge shows no pressure building up.

    5. Protect components and braze. Wrap valves, sight glasses and sensors with a wet cloth or heat-sink paste. Heat the joint evenly and let the filler run into the joint.

      ✓ Check: The filler forms a complete fillet all the way round.

    6. Keep the nitrogen on until cool. Keep the nitrogen flowing until the joint has cooled enough to touch.

      ✓ Check: When a joint is cut open, the inside is bright without black scale.

    7. Test the work. Pressure test with nitrogen, leak test, evacuate and record the work in the logbook.

      ✓ Check: The joint holds the test pressure and the circuit reaches the target vacuum.

Checks from the manufacturers’ manuals (10)

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

  • LG Multi V 5: error 151 means high and low pressure stay too close together, linked to a defective or disconnected four-way valve; three such errors within an hour lock the unit until a manual restart.[2]
  • Mitsubishi PLA-RP: check code PL (refrigerant circuit abnormal in cooling) is cleared only by switching the power off, and the service manual names the four-way valve as the part to replace.[3]
  • Mitsubishi SUZ-KA: first measure the reversing valve coil resistance (an open coil or unplugged connector can show voltage without a signal). Then, with the compressor disconnected, start a heating test run: 3 minutes after power-on the coil connector should have 230 V AC. Voltage present but the unit still cools means the valve; no voltage means the board.[4]
  • Temperzone UC8: Check which coil is which: in cooling the indoor coil should read close to the evaporating temperature, in heating close to the condensing temperature. If it is the wrong way round for more than a few minutes at over 50 % capacity with more than 20 K lift, the reversing valve is in the wrong position.[5]
  • LG Multi V 5: the four-way valve is de-energised in cooling, defrost and oil recovery and energised only in heating; it changes over during the 3-minute restart when the mode changes. Check the position by feeling the low-pressure service valve, and measure the coil insulation with a 500 V megger.[6]
  • Temperzone TZT-100 thermostat: Some thermostats keep the reversing valve energised for up to 2 hours after a heating or cooling call ends, to avoid the decompression hiss; that is normal and not a stuck valve.[7]
  • Toshiba RAV-GM***1ATP-E: Measure the four-way valve coil and check that it gets voltage when the unit switches mode; on Toshiba RAV-GM outdoor units the coil should read about 7.1 Ω ±5 % below 20 °C. Compare pipe temperatures in cooling and heating to confirm the valve actually moves.[8]
  • Toshiba RAS split units, Actron Air Advance: Measure the four-way valve coil: on split units it is typically about 1.7–2 kΩ (Toshiba about 1725 Ω ±10 % at 20 °C, Actron Air 2.03 kΩ at 25 °C). An open coil gives no switching at all.[9]
  • Fujitsu split air conditioners and heat pumps: Before suspecting the four-way valve on an older Fujitsu system with wired remote, check the remote DIP switch 5: on gives automatic changeover, off means the mode must be changed by hand. The small heat pump models 7–14 have no automatic changeover at all.[10]
  • Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II four-way valve (connector CN105): the coil is about 1.7 kΩ and is energised with rated AC in heating, not in cooling. Confirm the valve position from the pipe temperatures (hot gas to the outdoor coil in cooling, to the gas line in heating); if the coil is good and powered but the position is wrong, replace the valve.[11]
📝 Fill in a service report

Terms in this article

Brazing with nitrogen

Sources

  1. ACHR News – Troubleshooting heat pumps stuck in one mode
  2. LG – Multi V 5 outdoor units service manual, SM_MultiV_5_OutdoorUnits_09_18 (2018)
  3. Mitsubishi Electric – Service manual PLA-RP35–140BA, OCH416 revised edition D (September 2012)
  4. Mitsubishi Electric – Technical & service manual OCH545 (SUZ-KA25–71VA4), PDF p. 29–30 (11-5 A–E)
  5. Temperzone – UC8 Troubleshooting Guide, issue 2 (27 July 2015), p. 14
  6. LG – Multi V 5 Outdoor Unit Service Manual (09/18), Checking the High/Low Pressure Sensors, p. 122
  7. Temperzone – Service Training Manual, Issue 08, January 2023, pp. 283–285
  8. Toshiba – RAV-GM***1ATP-E (R32) service manual SVM-18043-8, p. 69 (P19 flow chart) and p. 74
  9. Toshiba – Service Manual Split Type, File No. SVM-18049-5 + Actron Air – Advance Split Ducted Troubleshooting Guide 9590-2015-03 Ver. 3, 11-9-4 Outdoor unit, p. 108; 04.08 EEV resistance
  10. Fujitsu – Fujitsu Trouble Shooting Guide (UK edition, undated, R22 era), p. 2 (settings: Auto Changeover DIP SW5) and p. 3 (Wiring) (undated; copy archived 2026-11-02)
  11. Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 12, p. 04-96 (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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