Mitsubishi Electric air conditioning systems display error codes on the indoor unit when a fault is detected. Understanding what these codes mean is the first step to diagnosing a fault and deciding whether it is something you can address yourself or whether you need to call an engineer. Here is a comprehensive guide to the most common Mitsubishi Electric error codes we encounter across our service calls in Surrey, Sussex and South London.
How to Read Mitsubishi Electric Error Codes
On most Mitsubishi Electric indoor units, error codes are displayed on the unit’s LED display or through a flashing pattern on the operation and timer indicator lights. The display typically shows a two-character code consisting of a letter followed by numbers — for example E1, E3, P1, U1, and so on. Some units display the code continuously; others flash a pattern that requires counting the flashes to identify the code. Refer to your installation manual for the specific display method on your model.
E-Series Codes — Electronic/Communication Faults
E0: Signal transmission fault — the indoor unit cannot communicate with the remote control. Check that batteries in the remote are fresh and that there is no obstruction between the remote and the indoor unit receiver. If the fault persists with new batteries and clear line of sight, the remote control or indoor unit receiver may need replacement.
E1: Communication fault between indoor and outdoor units. Check that the inter-unit communication cable is securely connected at both ends. This fault can also be caused by electrical interference or a failed component on either the indoor or outdoor unit PCB. Requires engineer attendance.
E3: Indoor unit fan motor fault. The indoor fan has stopped or is running at incorrect speed. Can be caused by a failed fan motor, a fault on the indoor PCB or a loose motor connection. Requires engineer attendance. Continuing to run the system with this fault can cause the evaporator coil to ice up.
E4: Remote controller communication error. The system cannot communicate with a wired remote controller (where fitted). Check controller wiring connections. May indicate a failed controller or PCB fault.
E5: Outdoor unit overcurrent protection — the outdoor unit is drawing excessive current. Causes include compressor problems, electrical supply issues or a fault on the outdoor PCB. Switch the system off at the isolator and call an engineer. Do not continue to operate the system with this fault showing.
E6: Communication fault between indoor units in a multi-split installation. The lead indoor unit cannot communicate with one or more follower units. Check inter-unit wiring. Requires engineer investigation.
P-Series Codes — Temperature Sensor Faults
P1: Indoor unit intake air temperature sensor fault. The thermistor measuring room air temperature at the indoor unit has failed or its connection has become loose. The system may still operate but temperature control accuracy will be compromised. The thermistor is a low-cost replacement part.
P2: Indoor unit pipe temperature sensor fault. The thermistor measuring the temperature of the refrigerant pipe has failed. Requires replacement — relatively straightforward repair.
P4: Drain water level sensor fault — the condensate level in the drain pan has risen to the overflow level. The most common cause is a blocked condensate drain pipe. Check the drain pipe for blockages. If the drain is clear and the fault persists, the sensor itself may need replacement.
P5: Drain pump fault (where a condensate pump is fitted). The condensate pump is not operating. Check power supply to the pump and check that the condensate tray is not overflowing. Pump replacement may be required.
P6: Freeze-up protection — the indoor unit has detected that the evaporator coil is approaching freezing point and has shut down cooling to prevent damage. The most common cause is a dirty or blocked air filter restricting airflow over the coil. Clean the filter, allow the ice (if present) to defrost, and restart. If the fault recurs after filter cleaning, low refrigerant charge or a failing fan motor may be the cause.
U-Series Codes — System/Refrigerant Faults
U1: Reversed phase or phase loss (three-phase systems only). Not applicable to single-phase domestic systems.
U2: Low voltage protection — the electrical supply voltage has dropped below the minimum operating level. Check your electrical supply. If voltage is normal, the fault may be within the system’s power supply circuit.
U3: Discharge temperature too high — the compressor discharge temperature has exceeded safe limits. Causes include high ambient temperature combined with high load, low refrigerant charge, or a failing compressor. Do not continue to operate the system. Call an engineer.
U4: Communication fault between indoor and outdoor units. Similar to E1 — check inter-unit cable connections. If cable is secure, PCB fault likely.
U6: Compressor overcurrent or IPM fault. The compressor power module has detected a fault condition. Switch off immediately and call an engineer. This code can indicate a failing compressor or inverter board fault.
U8: Outdoor unit thermistor fault — temperature sensors on the outdoor unit have failed or developed a fault. Requires engineer attendance for sensor testing and replacement.
What to Do When You See an Error Code
For P1, P2 sensor faults and P6 freeze-up after filter cleaning — these are relatively minor issues that may resolve with basic maintenance. For E-series communication faults and U-series system faults, switch the system off at the isolator and call an engineer. Continuing to run a system with compressor, motor or communication faults can cause progressive damage.
We carry common Mitsubishi Electric replacement parts including thermistors, PCBs and fan motors. For most fault code repairs we achieve a first-time fix. Call 07833 053749 or contact us online.