P8 fault code on a Mitsubishi Electric unit

As Adelaide’s largest independent service and repair company in Adelaide, we have seen a lot of Mitsubishi Electric air conditioning systems. One of the most common issues we’ve 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?

As we mentioned above, 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

Rectifying an outdoor unit fault on a Mitsubishi Electric unit

Depending on the fault you find on the outdoor PC Board will determine how to rectify the issue. These tests are not DIY and will require a Mitsubishi Electric specialist to test correctly.

Disclaimer - Electricity is dangerous and can kill you. The following information is for educational purposes only. Tests must be performed by an authorised service provider or Mitsubishi Electric specialist. Do not try this to perform these test without the proper expertise and qualifications.

Reverse Phase Detect/Open Phase Detect

This means you have a 3 phase system installed and 1 of the phases are missing OR wired the wrong way. Check for 3 phase power to the unit and rectify if 1 or more phases are missing. 2 phases may need to be swapped to ensure the correct compressor rotation.

Pipe Sensor Open/Short

This means 1 or more of the sensors is registering as open or short circuit. You will need to measure the resistance of each outdoor sensor to determine which is faulty.

HP Switch(63H2)Operation

HP or High pressure safety means the unit has measured a dangerously high pressure at the outdoor unit. This can be caused by various problems including a lack of airflow from the indoor unit (on heating) or the outdoor unit (on cooling). A HP fault can also be caused by a faulty HP switch, contaminated refrigeration circuit, faulty fan motor/s etc.

O/L (51CM) Operation

O/L or overload is present if the compressor draws excessive current (amps). This can be caused by a seized compressor, contaminated refrigerant, faulty compressor contactor, bad input voltage or anything that can put excessive load on the compressor.

Thermal Switch (26C) Operation

Discharge thermistor fault. This fault means the discharge line has reached an abnormally high temperature. This fault can be caused by a lack of refrigerant or possibly a faulty discharge thermistor.

Thermistor(TH3)Overheat Protection

Defrost sensor is open or short circuit. Check the resistance of the defrost sensor and replace if it’s not within correct range.

 Defective Input

Outdoor PC Board electronics fault. This fault is rarely ever seen. We have not come across a Mitsubishi Electric unit with this fault displayed…...yet!

Costs involved to repair your air conditioning system

It is important to ensure the correct tests are completed to determine what part requires replacement. It can be an expensive exercise to replace a working component when the fault could actually be an energy supplier issue. Resolving a P8 fault could be as little as $140.00 to fix a power supply issue. For a refrigerant leak, cost could be upwards of $2,000.00 especially if the repair is time consuming to fix. Ensuring the correct parts are diagnosed is vital to make certain you do not waste money on unnecessary parts or repairs.

In a nutshell

  • P8 faults can be caused by almost anything but is usually due to an outdoor unit issue
  • Finding the problem will involve checking the outdoor unit PC Board
  • Correctly determining the faulty part can save you $1,000’s
  • Using a Mitsubishi Electric specialist will ensure the problem is correctly diagnosed and repaired

Need help?

T&K Airpower have been working to keep Adelaide homes comfortable for the last 20 years. We specialise in Temperzone, Hitachi and Mitsubishi Electric air conditioning units and know how they tick. Do you have a P8 fault displayed on your Mitsubishi Electric air conditioning unit? Give us a call and we will be more than happy to help.

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