Error fault code on Mitsubishi Electric ducted air conditioning systems

UF / UP / U6 fault code on Mitsubishi Electric ducted systems

A common fault on Mitsubishi Electric ducted systems is the UF, UP or U6 error code. This fault code commonly represents a fault within the Outdoor Power PCB, Compressor electrical circuit or Compressor bearings. Of these 3 similar errors codes, the most common to see is the U6, UP or UF. This will be displayed on your wall controller after the unit tries to run 10 times in a row without success.


Mitsubishi Electric define UF / U6 / UP faults as follows:

UF – Compressor overcurrent interruption (when compressor locked). Abnormal if overcurrent of DC bus or compressor is detected within 30 seconds after compressor starts operating.

U6 – Power module abnormal. Check abnormality by driving power module in case overcurrent is detected. (UF or UP error condition).

UP – Compressor overcurrent interruption. Abnormal if overcurrent DC bus or compressor is detected after compressor starts operating for 30 seconds.

Common issues that can cause U6 / UF / UP faults

  • Failure or the Power PCB in outdoor unit
  • Electrical failure or fusion of Compressor
  • Seized bearings in the Compressor
  • Disconnected/broken wiring to the compressor
  • Closed service valves at outdoor unit
  • Low voltage at outdoor unit
  • Tests to determine faulty components


P8 fault code on a Mitsubishi Electric unit

One of the most common issues seen with older fixed speed Mitsubishi Electric air conditioning units is the P8 fault. The P8 fault is quite generic and can be the front for a multitude of different issues.


What causes the P8 error code?

The P8 fault can mean many things when it pops up on your Mitsubishi Electric wall controller. The P8 fault is activated when the indoor coil sensor does not change temperature for a set amount of time after the compressor is (or should be) switched on. Put simply, if the indoor unit does not get hot or cold within a predetermined time frame (approx. 9 minutes), the indoor unit thinks the compressor is not running and faults on P8. The reason for the compressor not running can be anything. To further diagnose the unit you will need to look at the outdoor unit PC Board.


What can a P8 fault mean on your Mitsubishi Electric air conditioning system?

On older fixed speed Mitsubishi Electric units, the P8 error code generally points to a fault with the outdoor unit. The outdoor faults on your Mitsubishi Electric unit will be displayed on the Outdoor PC Board. These faults are shown via small red LED lights on the PC Board. If the lights are Solid, this is not a fault and represents the units operating condition. If the light is flashing, a fault is present.

The following faults/conditions are represented by the numbered LED lights on the Outdoor PC Board:

Solid (No fault) Flashing (Fault)
LED 1 – Compressor Command Reversed Phase Detect
LED 2 – Heat Command Open Phase Detect
LED 3 – During 63H1 Operation Pipe Sensor Short/Open
LED 4 – Compressor ON HP Switch(63H2)Operation
LED 5 – Outdoor Fan ON O/L (51CM) Operation
LED 6 – 4 Way Valve ON Thermal Switch (26C) Operation
LED 7 – Bypass Valve ON Thermistor (TH3) Overheat Protection
LED 8 – Crankcase heater ON Defective Input


You can find more information by visiting the Mitsubishi Electric support website >

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