Quick answer
Mitsubishi error code P8 means abnormality of pipe temperature: the indoor pipe temperature is not changing the way the heating or cooling mode demands. The system shuts down on this code, so do not keep restarting it. Confirm the outdoor unit has power at the breaker and disconnect and is clear of snow and ice, then power-cycle once. If P8 returns, call a licensed technician.
Mitsubishi lists error code P8 as an abnormality of pipe temperature, which means the indoor pipe temperature is not changing the way the selected mode demands. In cooling, the pipe at the indoor coil should turn cold; in heating, it should turn hot. When the indoor unit sees neither, it stops and posts P8. This definition is from the self-diagnosis table for the PAR-21MAA controller on Mr. Slim systems, and your model's manual and label are the reference if they differ.
What “pipe temperature” means on the indoor unit
A small sensor called a pipe thermistor sits in a holder against the refrigerant pipe of the indoor coil. If refrigerant is circulating properly, that pipe changes temperature soon after the compressor starts. If it does not, the unit concludes the refrigerant circuit is not doing its job.
So P8 is a refrigerant-circuit code reported from indoors. The cause can be the refrigerant, the valves, the outdoor unit, or the sensor that is supposed to be watching.
Why you should not keep restarting a system on P8
The system shuts down on this code, and that shutdown is protective. If the cause is a closed valve or a low refrigerant charge, every restart runs the compressor against a problem it cannot overcome. One power cycle is a fair test. Repeated restarts put the most expensive part of the system at risk.
Check outdoor power and snow, then power-cycle once
- Look at the outdoor unit. If it is silent and the fan is still while the indoor unit calls for heating or cooling, it may have no power.
- Check the breaker in the panel and the disconnect switch on the wall beside the outdoor unit.
- In winter, clear snow from around and under the unit and look for ice covering the coil. Do not chip at the fins.
- Turn the breaker off, wait a few minutes, and turn it back on.
- Run the system. If P8 returns, switch it off and leave it off.
Snow matters because a buried outdoor unit cannot exchange heat with the outside air. What to do if your heat pump is covered in snow has the safe clearing method.
Causes of P8, and how the system's age changes the order
On a system that has worked for years, start with refrigerant and power. On a new or recently serviced system, start with valves and connections.
- Low refrigerant. A heat pump does not use up refrigerant, so a low charge means a leak somewhere in the piping or coils.
- An outdoor unit that is not running, whether from lost power or a fault of its own.
- An indoor pipe thermistor that has come out of its holder and is reading air instead of the pipe.
- A closed stop valve. The stop valves on the outdoor unit are opened at installation; one left shut blocks refrigerant flow.
- Crossed piping or wiring on a multi-zone system, where the head being controlled is not the head receiving refrigerant.
The crossed-connection case has a recognisable pattern: calling for heat in one room makes a different room's head warm up. If that describes a newly installed multi-zone system, tell the installer exactly that. Why a mini-split is not heating walks through the no-heat causes that do not involve a code.
Why the P8 repair is licensed work in Ontario
Thermistor, stop-valve and refrigerant checks are licensed work. Anyone handling refrigerant in Ontario must hold an ODP card, and topping up a leaking system without finding the leak is not a repair. A technician confirms the thermistor's position and reading, verifies the stop valves, checks operating pressures, and leak-tests if the charge is short.
- On-site diagnosis: $90–$150, applied to the repair when you go ahead
- Refrigerant leak repair and recharge: $300–$800+, with finding and sealing the leak making up most of it
- Thermistor replacement or re-seating: quoted in writing after testing
- Multi-zone piping or wiring corrections: quoted in writing, and on a new install normally a matter for the installer
ZK Mechanical services Mitsubishi systems across Burlington, Oakville, Milton, Hamilton, Mississauga and Georgetown. If P8 came back after one power cycle, book heat pump repair or contact us with the model number and the code. The Mitsubishi error code guide lists related codes, including E6 for communication faults.