Mitsubishi Electric Error Codes

Mr. Slim and City Multi systems display codes on the wired controller or by flashing LEDs on the outdoor PCB.

Code	Description	Likely cause	Recommended action
P1	Indoor intake (room) thermistor fault	Open / short / out-of-range room sensor	Check connector CN20, measure thermistor resistance, replace sensor or indoor PCB.
P2	Indoor pipe (liquid) thermistor fault	Faulty TH2 pipe sensor or wiring	Inspect TH2 wiring and connector, test resistance vs temp chart, replace sensor if open/short.
P4	Drain sensor / drain pump fault	Float switch tripped or drain pump failed	Clear drain pan and trap, test float switch, replace drain pump if seized.
P5	Drain overflow protection	Blocked condensate drain	Clear blockage, check fall on drain line, verify pump operation.
P6	Freeze / overheat protection	Low airflow, dirty filter or low charge	Clean filters and coil, check fan speed, verify refrigerant charge.
P8	Outdoor pipe temperature abnormal	Refrigerant under/overcharge or restricted flow	Check charge, EEV operation and pipe sensors.
E0	Remote controller transmission error	Wired controller wiring or address fault	Check controller cable polarity and main/sub setting.
E3	Remote controller signal receiving error	Broken communication between indoor/outdoor	Check S1-S2-S3 interconnect, supply voltage and outdoor PCB.
E6	Indoor/outdoor unit communication error	Wiring fault or PCB failure	Verify S1-S2-S3 continuity and 24 V DC between S2-S3.
E9	Indoor/outdoor communication error (other side)	Outdoor reports loss of comms with indoor	Check interconnect, indoor power and PCB.
U2	Compressor overheat / discharge temperature high	Low refrigerant, blocked condenser, EEV fault	Check charge, clean condenser, test EEV and discharge sensor.
U4	Outdoor thermistor open/short	Faulty TH3/TH6/TH7/TH8 sensor	Identify sensor by sub-code, measure resistance, replace.
U6	Compressor overcurrent / power module fault	Stalled compressor, IPM failure	Measure compressor windings, inspect IPM, check DC bus.
U7	Superheat abnormal due to low discharge temp	Liquid back, overcharge or EEV stuck open	Verify charge and EEV stepping.
UF	Compressor overcurrent at start-up	Locked rotor, low voltage, IPM fault	Megger compressor, check supply voltage and inverter PCB.
FB	Indoor controller fault	EEPROM or PCB failure	Power-cycle; replace indoor PCB if persists.

Indoor intake (room) thermistor fault

Cause:: Open / short / out-of-range room sensor
Action:: Check connector CN20, measure thermistor resistance, replace sensor or indoor PCB.

Indoor pipe (liquid) thermistor fault

Cause:: Faulty TH2 pipe sensor or wiring
Action:: Inspect TH2 wiring and connector, test resistance vs temp chart, replace sensor if open/short.

Drain sensor / drain pump fault

Cause:: Float switch tripped or drain pump failed
Action:: Clear drain pan and trap, test float switch, replace drain pump if seized.

Drain overflow protection

Cause:: Blocked condensate drain
Action:: Clear blockage, check fall on drain line, verify pump operation.

Freeze / overheat protection

Cause:: Low airflow, dirty filter or low charge
Action:: Clean filters and coil, check fan speed, verify refrigerant charge.

Outdoor pipe temperature abnormal

Cause:: Refrigerant under/overcharge or restricted flow
Action:: Check charge, EEV operation and pipe sensors.

Remote controller transmission error

Cause:: Wired controller wiring or address fault
Action:: Check controller cable polarity and main/sub setting.

Remote controller signal receiving error

Cause:: Broken communication between indoor/outdoor
Action:: Check S1-S2-S3 interconnect, supply voltage and outdoor PCB.

Indoor/outdoor unit communication error

Cause:: Wiring fault or PCB failure
Action:: Verify S1-S2-S3 continuity and 24 V DC between S2-S3.

Indoor/outdoor communication error (other side)

Cause:: Outdoor reports loss of comms with indoor
Action:: Check interconnect, indoor power and PCB.

Compressor overheat / discharge temperature high

Cause:: Low refrigerant, blocked condenser, EEV fault
Action:: Check charge, clean condenser, test EEV and discharge sensor.

Outdoor thermistor open/short

Cause:: Faulty TH3/TH6/TH7/TH8 sensor
Action:: Identify sensor by sub-code, measure resistance, replace.

Compressor overcurrent / power module fault

Cause:: Stalled compressor, IPM failure
Action:: Measure compressor windings, inspect IPM, check DC bus.

Superheat abnormal due to low discharge temp

Cause:: Liquid back, overcharge or EEV stuck open
Action:: Verify charge and EEV stepping.

Compressor overcurrent at start-up

Cause:: Locked rotor, low voltage, IPM fault
Action:: Megger compressor, check supply voltage and inverter PCB.

Indoor controller fault

Cause:: EEPROM or PCB failure
Action:: Power-cycle; replace indoor PCB if persists.

For field guidance only. Codes and procedures vary by model and firmware — always confirm with the Mitsubishi Electric service manual for the specific unit before carrying out a repair.

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