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 the 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 unit’s 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 to perform these tests 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 is 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). An 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 it if it’s not within the 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 in repairing 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, the 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
- Using a Mitsubishi Electric specialist will ensure the problem is correctly diagnosed and repaired
Frequently Asked Questions
What does the P8 fault code mean on a Mitsubishi Electric air conditioner?
The P8 fault is triggered when the indoor coil sensor does not detect a temperature change within about 9 minutes of the compressor being switched on, so the indoor unit assumes the compressor is not running. It is a generic fault on older fixed-speed Mitsubishi Electric units and usually points to a problem at the outdoor unit rather than the indoor unit.
What causes a P8 fault?
Because P8 simply means the system is not heating or cooling as expected, the underlying cause can be almost anything at the outdoor unit. On older fixed-speed units the specific fault is shown by the LED lights on the outdoor PC board, and can include a reversed or open phase on a three phase supply, a pipe sensor open or short circuit, a high pressure trip (often from poor airflow or a dirty coil), compressor overload, a discharge thermistor fault from low refrigerant, or a defrost sensor fault.
Can I fix a P8 fault myself, or do I need a technician?
A P8 fault should be diagnosed and repaired by a licensed air conditioning technician, not as a DIY job, because the checks involve the unit’s electrical and refrigeration components and mains power. As authorised Mitsubishi Electric warranty agents in Adelaide, we diagnose the exact cause first, then quote a fixed price before any work starts. If your system is still under manufacturer warranty, we can assess whether the fault qualifies and handle the warranty process directly. Call 08 8296 7888 to book a diagnostic.
How much does it cost to repair a P8 fault?
It depends entirely on what is actually causing the fault, which is why correct diagnosis matters. A P8 caused by a power supply issue can be as little as around $140 to fix, while a refrigerant leak can run upwards of $2,000 if it is time consuming to repair. Diagnosing the real fault first means you do not waste money replacing parts that are not faulty.
