45 McQuay air conditioner error codes + troubleshooting

Is your McQuay air conditioner suddenly blowing warm air, shutting down entirely, or flashing a cryptic alphanumeric code like U4, L5, or E7 on the digital display? During extreme temperatures, a malfunctioning AC feels like an emergency, but those flashing letters and numbers are actually your system’s built-in self-diagnosis tool pinpointing the exact mechanical or electrical failure.

Because McQuay systems utilize advanced Daikin-engineered internal architecture, their diagnostic matrix is highly sophisticated. It separates basic system warnings like low refrigerant (U0) or a clogged drain pump (A3) from critical commercial-grade failures like Intelligent Power Module overloads (L5) and complex thermistor short circuits (J-Series). Decoding these McQuay faults quickly can save you from unnecessary panic and expensive commercial service calls.

Instead of scrolling through dense, hard-to-find OEM troubleshooting manuals, use our Official McQuay AC Error Code Decoder below. Simply select your code from the complete 45-code matrix to instantly reveal the hidden cause and the step-by-step DIY solution.

⚠️ Pro Safety Tip: While issues like cleaning an overflowing drain pan to clear a water level warning (A3) or power-cycling to clear a temporary glitch are safe DIY fixes, errors involving IPM high-voltage lockouts (L5), open phases in the power supply (P1), or high-pressure switch failures (E3) should always be handled by our licensed HVAC technicians to prevent severe electrical hazards.

McQuay air conditioner error code U0

Causes:

1. This error is reported when refrigerant or cooling gas is insufficient in the unit.

Solutions:

Contact a local technician to visit the home and:

1. Check the harness for any pinched wire or loose connections. Repair as required.
2. Check the sensor for any faults. Replace if needed.
3. Check the stop valve and shut-off valve also known as EXV for any clogging. Replace the valve if broken.
4. Check the main PCB and replace it if it’s malfunctioning.

McQuay air conditioner error code U2

Causes:

1. This error occurs when DC voltages are out of range or not consistent.

Solutions:

1. Switch OFF the air conditioner for about 5-10 seconds and then turn it ON again.
2. Using a voltage meter, check the supply voltages and make sure they are enough and consistent.
3. Open the outdoor unit and rotate the fan. Remove if something is causing a clog or stuck in the fan.
4. Take the services of a local technician to check the power supply waveform and the power supply itself for any faults. Repair and replace components as needed.

McQuay air conditioner error code U4

Causes:

1. This error is reported when one component or part of the air conditioner is unable to communicate with other components.

Solutions:

1. Using a voltage meter, check the voltages of the signal terminal for inconsistency.
2. Check the waveform of the main power supply to the unit for any disturbance.
3. Manually move the fan of the outdoor unit and make sure there isn’t any resistance.
4. Call a technician to check the wiring, connections, and connectors of indoor and outdoor units. Repair harness as needed.

McQuay air conditioner error code U7

Causes:

1. This error is reported when the main PCB (Printed Circuit Board) of the outdoor unit detects a signal transmission issue.

Solutions:

1. Turn OFF the air conditioner and then turn it ON again after a few seconds.
2. Seek professional help to inspect outdoor PCB for any malfunctioning. Replace the PCB if it is faulty.
3. If still getting the error, inspect the whole of the outdoor unit as this can be a long-term monitor or technical issue.

McQuay air conditioner error code UA

Causes:

1. This error indicates some issues in the installation.
2. Indoor and outdoor units are not of the same model or size.

Solutions:

1. Call a technician to check the main PCB for any malfunction. Replace the PCB if it is faulty.
2. Look at the label for part code on indoor and outdoor units and ensure that both are of the same model and size.
3. Seek the assistance of a local installation operator to check the installation.
4. In case of wrong units, call the company’s representative for unit replacement.

McQuay air conditioner error code UF

Causes:

1. This error occurs when the indoor and outdoor units are unable to communicate properly.
2. The wiring and piping between indoor and outdoor units are damaged or broken.

Solutions:

1. Check the wiring for any damage and the piping for any leaks. Repair the harness and piping where required.
2. Check the gas or refrigerant level in the air conditioner. Get it filled if it’s insufficient.
3. Call a technician to check the refrigerant line for any clogging.

McQuay air conditioner error code UH

Causes:

1. This error is reported by the air conditioner when the antifreeze functions have been activated.

Solutions:

1. Wait for a few minutes as this error is mostly tackled automatically by the AC unit itself.
2. Using a voltage meter, check the amount of supply voltages. Make sure they are consistent.
3. Look at the label of both units and ensure that they are of the same model and size by matching part codes.
4. If still getting the error, contact a technician to check the power supply and main PCB for any faults. Replace either of the components if needed.

McQuay air conditioner error code A1

Causes:

1. The A1 error code is a direct indication of some issues with the main PCB of the indoor unit.

Solutions:

Contact a local professional to visit your house and:

1. Check the wiring, connections, and connectors of indoor PCB for any looseness or breakages. Repair the harness as required.
2. Check the PCB for malfunctioning. Replace the PCB if it’s faulty.

McQuay air conditioner error code A3

Causes:

1. This error is reported when water is not draining out of the air conditioner.

Solutions:

1. Restart the air conditioner after turning it OFF for a few seconds.
2. Check the water level to be drained as AC may not drain out just because the water is too low.
3. Check and ensure that the float switch is working fine and not activated when it shouldn’t.
4. Check the wiring and connections of the drain pump with the main PCB and other power components.
5. Check the drain pump for an open or short circuit. Replace the components if needed.

McQuay air conditioner error code A5

Causes:

This is an antifreeze or defrosts error that can occur due to multiple reasons.

1. Breakages in wiring and leakages in piping between indoor and outdoor units.
2. EXV is malfunctioning.
3. Dirt and debris have covered some components of the AC.
4. Some of the valves are clogged, faulty, or completely broken.

Solutions:

1. Check the air passage of the air conditioner for any blockages or restrictions.
2. Inspect the intake filter of the unit. Remove dirt and debris build-up.
3. Check and remove the dust if it has accumulated in the indoor coils and cooling coils.
4. Inspect the wiring for any damages, and check the piping for any leakages. Repair wiring and piping as needed.
5. Check the EXV for any issues. Repair and replace the valve as needed.
6. Check the resistance coefficient of the indoor coil sensor and room sensor.
7. Check the refrigerant level for insufficiency and the refrigerant line for any issues. Fill the gas and repair the line if faulty.

McQuay air conditioner error code A6

Causes:

1. This error indicates some issues or abnormalities in the fan motor of the outdoor unit.
2. Something is stuck in the fan of the outdoor unit.
3. Voltages to the fan motor are insufficient or not stable.

Solutions:

1. Check the indoor fan and rotate it manually to ensure there are no restrictions. Replace the fan motor if it’s faulty.
2. Using a meter, check to be sure that enough and consistent voltages are being supplied to the indoor fan motor.
3. Check the capacity level of the AC motor and fan motor. Replace the capacitor if there is conductivity.
4. If still getting the error, check the harness and main PCB for any malfunctioning, and replace the board if faulty.

McQuay air conditioner error code C4

Causes:

1. This error indicates an open or short circuit in the heat exchanger of the indoor unit. 

Solutions:

1. Using a tester, check the resistance coefficient of the sensor and if it is weak, replace it with a compatible one.
2. Call an authorized service provider or a local engineer to check the sensor for any open or short circuit. Replace the sensor if faulty.
3. If still getting the error, inspect the main PCB, harness, or control board of the indoor unit. Repair or replace elements as needed.

McQuay air conditioner error code C9

Causes:

1. This error indicates an open or short circuit in the room thermistor of the indoor unit. 

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “C4”.

McQuay air conditioner error code E1, L1

Causes:

1. This error is directly linked to some issues in the main PCB of the outdoor unit.

Solutions:

1. Using a meter, check the voltages range towards and from the power supply of the unit.
2. Check the wiring and connections between the compressor and main PCB. Repair harness as required.
3. Check the resistance coefficient of the fan motor of the outdoor unit.
4. Check the waveform of the power supply and make sure it is stable.

McQuay air conditioner error code E3

Causes:

1. This error indicates that the air conditioner has initiated high-pressure protection.

Solutions:

1. Turn OFF the air conditioner and then turn it ON again after a few seconds.

Call a technician to,

• Check the stop valve.
• Using a pressure meter or gauge, check the pressure level.
• Check the HPS connection for malfunction and make sure it is consistent.

McQuay air conditioner error code E4

Causes:

1. This error indicates low pressure in the system.
2. Low-pressure protection has been activated in the air conditioner.

Solutions:

1. Check the stop valve for any clogging or fault. Replace the valve if broken.
2. Check the resistance coefficient of the sensor and if it’s weak, replace it.
3. Check the wiring and connections to the low-pressure sensor and coil sensor of the outdoor unit. Replace either of the sensors if faulty.
4. Check voltages to the sensor and ensure a consistent and stable supply.
5. If you’re still getting the error, the refrigerant level is low. Call an engineer to fill the freon.

McQuay air conditioner error code E5

Causes:

1. This error is an indication that there is too much load on the compressor motor.
2. The compressor motor is locked.
3. Low voltages to the compressor motor.

Solutions:

1. Check the wiring and connections between the discharge pipe sensor and the main PCB. Replace the sensor if it’s faulty.
2. Check the resistance of the discharge pipe sensor and if it’s weak, replace it.
3. Check the EXV valve.
4. Ensure that the refrigerant line is smooth and doesn’t have any clogging or restrictions.

McQuay air conditioner error code E6

Causes:

1. This error is reported when the compressor is locked.
2. The compressor is unable to start.

Solutions:

1. Check the system with a specified inverter checker.
2. Inspect the EXV valve for any blockages or breakages.

McQuay air conditioner error code E7

Causes:

1. This error pop-up on the screen when the DC fan motor is locked or overloaded in the outdoor unit.

Solutions:

1. Check the harness of the fan motor for any loose or broken connections.
2. Take a thorough look for any foreign items stuck in or around the fan. Remove such objects and move the fan manually for one time.

McQuay air conditioner error code E8

Causes:

1. This is an AC input over current and occurs because of some ups and downs.

Solutions:

1. Using the meter, check the input current for continuity.
2. Check the electrolytic capacitor of the main circuit. Replace the capacitor if needed.
3. Check the pressure and ensure that everything is installed properly.
4. Check the whole system using a specified inverter checker.

McQuay air conditioner error code E9

Causes:

1. This error indicates some issues with the EXV valve of either of the air conditioner units.

Solutions:

1. Turn OFF the air conditioner and then turn it ON again after a few seconds.
2. Check the wiring and connections between the EXV and the main PCB of the air conditioner.
3. Check the resistance of the sensors and the EXV coil. Replace the sensor if needed.

McQuay air conditioner error code EA

Causes:

1. This error is a clear indication that the 4-way valve of the air conditioner is malfunctioning or completely broken.

Solutions:

Call a technician to visit your home and:

1. Check the wiring and connections of the 4-way valve.
2. Check the 4-way valve switch output and ensure it is continuous.
3. Check the connections and resistance of the sensor. Repair the connection and replace the sensor if faulty.
4. Make sure that nothing is causing a clog or restriction in the refrigerant line.

McQuay air conditioner error code F3

Causes:

1. This error occurs when the discharge pipe is overheating.
2. The discharge pipe sensor is giving the wrong measurements.
3. The refrigerant line has some issues.

Solutions:

1. Check the wiring and connections between the discharge pipe sensor and the main PCB. Repair harness as needed.
2. Check the discharge pipe sensor for any faults. Replace the sensor if broken.
3. Check the EXV valve for malfunction and clogging. Replace the valve if broken.
4. Call a technician to inspect the refrigerant line. Remove the object causing a restriction or blockage.

McQuay air conditioner error code F6

Causes:

1. This error is triggered on AC when there is a rise in heat exchanger temperature or overheating.

Solutions:

1. Call an installation engineer to ensure that components related to heat exchangers are in place.
2. Check the fan of the outdoor unit for any restrictions. Move it manually until it’s running smoothly.
3. Check the EXV for any blockages or clogging. Replace the valve if faulty.
4. Check the coils for dirt covering. Do a thorough cleaning to wipe off debris build-up.
5. Check the coil sensor for any malfunction. Replace the sensor if faulty.

McQuay air conditioner error code H0

Causes:

1. This error occurs when the compressor sensor system has some issues or is abnormal.

Solutions:

Take the services of a technician to:

1. Check and ensure that the reactor connection and compressor connection are fine and in place.
2. Using a gauge or meter, check the resistance between the reactor terminals and compressor terminals. Seek assistance from a technician for further guidance or repair.

McQuay air conditioner error code H3

Causes:

1. This error is linked directly to the high-pressure switch of the air conditioner.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “E3”.

McQuay air conditioner error code H6

Causes:

1. This error occurs on the air conditioner when there is some issue or abnormality in the position sensor of the unit.
2. Open or short circuits in the position sensor.

Solutions:

1. Call a technician to check the sensor for any open or short circuits. Replace the sensor if faulty.
2. Using a meter, check the voltages of the electrolytic capacitor. Replace the capacitor if needed.
3. Check and repair the compressor harness wire if it is pinched or broken.
4. Check the system with an inverter checker and repair it as suggested by the technician.

McQuay air conditioner error code H8

Causes:

1. This error is displayed on the screen when the AC current sensor is showing some issues.

Solutions:

1. Restart the air conditioner after turning it OFF for a few seconds.
2. Using a meter, check the capacitor and rectifier input voltages. Replace the capacitor if needed.
3. Check the harness for any pinched wire or loose connectors. Call a technician for repairs.

McQuay air conditioner error code H9

Causes:

1. This error is a clear indication that there is an open or short circuit in the air thermistor sensor of the outdoor unit.

Solutions:

1. Using a tester, check the resistance coefficient of the sensor and if it is weak, replace it with a compatible one.
2. Call an authorized service provider or a local engineer to check the sensor for any open or short circuits. Replace the sensor if faulty.
3. If still getting the error, inspect the main PCB, harness, or control board of the indoor unit. Repair or replace elements as needed.

McQuay air conditioner error code J1

Causes:

1. This error is reported when the pressure sensor of the air conditioner malfunctions.
2. Open or short circuit in the pressure sensor of AC.

Solutions:

1. Check the wiring and connections of the pressure sensor. Replace the sensor if it’s weak or faulty.
2. Check the voltages and pressure in the air conditioner. Seek assistance from a technician if required.

McQuay air conditioner error code J3

Causes:

1. There is an open or short circuit in the compressor discharge pipe thermistor.
2. The compressor discharge pipe thermistor is wrongly installed or not in the intended position.

Solutions:

1. Check the wiring and connection of the sensor to the main PCB.
2. Using a meter, check the resistance coefficient of the sensor and replace it if it’s weak.
3. Check the resistance of the indoor coil sensor. Replace the sensor if it’s faulty.

McQuay air conditioner error code J5

Causes:

1. This error is indicating an open or short circuit in the suction pipe thermistor of the AC unit.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “H9”.

McQuay air conditioner error code J6

Causes:

1. This error is reported when AC detects an open or short circuit in the heat exchanger of the outdoor unit.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “H9”.

McQuay air conditioner error code J7

Causes:

1. This error is reported when AC detects an open or short circuit in the subcooling heat exchanger of the AC unit.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “H9”.

McQuay air conditioner error code J8

Causes:

1. This error occurs when there is an open or short circuit in the liquid pipe thermistor of the AC unit.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “H9”.

McQuay air conditioner error code J9

Causes:

1. This error pop-up when the AC detects an open or short circuit in the gas pipe thermistor of the AC unit.
2. The gas pipe thermistor is malfunctioning and not working properly.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “H9”.

McQuay air conditioner error code LC

Causes:

1. This error occurs when the control PCB and inverter PCB of the air conditioner are unable to communicate properly.

Solutions:

1. Check the wiring and connection of the fan motor of the outdoor unit.
2. Check the LED light to see if it’s blinking on the outdoor unit PCB. If it is faulty, replace the board.

McQuay air conditioner error code L3

Causes:

1. This error is reported when there is a rise in the temperature of the electrical box.

Solutions:

1. Turn OFF the air conditioner and then turn it ON again after a few seconds.
2. Using a meter, check the resistance coefficient of the temperature sensor.
3. Call a technician to check the heat sink temperature and make sure it is in good shape and condition.
4. Ensure that the outdoor fan is not stuck and the fan motor is working fine.
5. Check and make sure that there is no error in the installation.

McQuay air conditioner error code L4

Causes:

1. This error indicates a temperature rise or overheating in the AC heat sink.

Solutions:

1. Turn OFF the air conditioner and then turn it ON again after a few seconds.
2. Check and ensure that the silicone grease in the heat sink is enough.
3. Check the resistance value of the temperature sensor and replace the sensor if weak or faulty.
4. Check and make sure that the outdoor fan is working fine and at full speed.
5. Call an installation engineer to ensure that heatsink and related components are installed properly.

McQuay air conditioner error code L5

Causes:

1. This error indicates breakages or issues in the harness and connections of IPM (Intelligent Power Module) or IGBT.

Solutions:

Seek the assistance of a local technician to come home and inspect the following components and factors such as:

1. Supply voltages.
2. Compressor phase.
3. Discharge pressure.
4. Stop valve.
5. Call an installation engineer and ensure that each component is placed in its intended position.

McQuay air conditioner error code L8

Causes:

1. This error indicates some issues with the electrical thermal switch of the air conditioner.

Solutions:

1. Contact the company representative or a local but experienced technician to check the switch and do further assistance.

McQuay air conditioner error code L9

Causes:

1. This is a stall prevention error in the air conditioner unit. 
2. Issues or misplacement in the components installation.

Solutions:

1. Check the stop valve for any malfunction. Replace the valve if needed.
2. Call a technician to ensure that high and low pressure is stable.
3. Check and make sure that the power transistor is being supplied consistently.
4. Check the output voltages using a meter. Contact an electrician to inspect the harness and power supply.

McQuay air conditioner error code P1

Causes:

1. This error indicates an open phase in the air conditioner.
2. Supply voltages are not enough or stable.

Solutions:

1. Check the LED light on the PCB and see if it’s malfunctioning. Replace the outdoor PCB if it is faulty.
2. Check and make sure that there isn’t an open phase in the power supply voltage. Call a technician if required.
3. Check and ensure that voltages between different phases are enough and stable.

McQuay air conditioner error code P4

Causes:

1. This error indicates some issues, open, or short circuits in the heat sink thermistor or air conditioner unit.

Solutions:

1. Try the same solutions as suggested for the McQuay Air Conditioner Error Code “H9”.

McQuay air conditioner error code PJ

Causes:

1. This error is reported when the capacitor or capacity settings are at fault.
2. The main PCB of the outdoor unit is malfunctioning.

Solutions:

1. Check the capacitor of the outdoor unit for damages. Replace the capacitor with a new and compatible one.
2. Call a technician to check the outdoor unit PCB for any issues. Replace the board if it is faulty.