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Mitsubishi Electric
Mitsubishi Electric uses a two-part system. P codes relate to the indoor unit or its sensors, and U codes relate to the outdoor unit or the relationship between indoor and outdoor. Once you know which side the code belongs to, you know which part of the system to look at, and that saves a lot of guessing before anyone picks up a meter.
| Prefix | What it covers | Where to look first |
|---|---|---|
| P0 | Indoor control board | Board itself, then supply voltage and connections |
| P1 and P2 | Fan motor and fan speed control | Fan free rotation, then motor and control circuit |
| P3, P6 and P5 | Temperature and humidity sensors | Sensor resistance against ambient, then plug and loom |
| P4 | Communication between indoor and remote or controller | Controller cable and terminations |
| U0 | Refrigerant leak detected | Pressure test before any top up |
| U1 and U2 | High and low pressure sensors | Sensor reading against a gauge, not against assumption |
| U3 and U4 | Indoor to outdoor communication, then outdoor board | Interconnecting cable first |
This catches people out constantly. On Mitsubishi Electric, U2 is a low pressure sensor fault. On Daikin, U2 is a voltage drop or overvoltage fault. U4 is an outdoor board fault here, and a transmission fault on Daikin. Never look up a code using another brand's chart, and never let a technician do it either. The chart has to match the badge on the outdoor unit.
The humidity sensor code shows up more often here than in drier climates because the indoor unit is working harder to dehumidify through summer. It gets blamed on the board far too often. The sensor is a cheap part and it is worth testing it properly before anything expensive gets replaced.
Do not reset the system repeatedly to clear a code. It will come back, and on pressure and compressor faults each restart does damage. Do not add refrigerant because a pressure code appeared, because the sensor itself may be the fault. Do not swap a board on a U3 or U4 code before the interconnecting cable and terminations have been tested, because that is usually where the fault actually is.
Treat these as urgent and stop using the system: a burning smell, a safety switch that trips every time the unit starts, water tracking near electrical fittings, or a compressor that tries to start and cuts out within seconds. Anything else can be booked and diagnosed properly.
P codes relate to the indoor unit. P0 is the indoor control board, P1 and P2 are fan motor related, P3 and P6 are temperature sensors, P4 is communication with the controller and P5 is the humidity sensor. The code narrows the fault to the indoor side, which is a useful start.
U3 is a failure of communication between the indoor and outdoor units. U4 is an outdoor control board fault. In practice U3 more often turns out to be the interconnecting cable, a loose termination or water in a connector, so test the cable and connections before condemning a board.
No, and a reputable technician will not. U0 means a leak has been detected. Topping up without finding and repairing the leak means paying for refrigerant that will escape again, and the system will log the same code within weeks. A pressure test comes first.
Because manufacturers do not share a code standard. Mitsubishi Electric U2 is a low pressure sensor fault. Daikin U2 is a voltage drop or overvoltage fault. Always match the chart to the brand and model, never assume a code behaves the same way across brands.
It is not dangerous, but it will affect dehumidification and could cause the unit to behave oddly in humid weather. P5 appears more in Sydney than in drier climates. The sensor is inexpensive and it is worth testing properly rather than replacing the control board on a guess.
You can clear it once to see whether it returns, which is useful information. Do not keep doing it. On pressure, compressor and communication faults, each restart can cause further damage, and repeated resets hide the pattern a technician needs to diagnose it.
It depends on the fault, the part and the access. A sensor or cable fault sits at the low end, a board or compressor at the high end. We diagnose first and give you a written figure before any work starts, so you are not paying for a guess.
On a multi head system each indoor unit has its own board and sensors, so a P code on one head is usually local to that head. A U code is more likely to affect all of them because it involves the outdoor unit or the shared communication path.
Yes, and we do it regularly. We will also tell you if something in the original installation is causing the repeat fault, such as undersized pipework, a poor drainage fall or a unit that was never commissioned properly.
The exact code, the model number from the indoor unit sticker, whether the code appears immediately or after running, and whether it happens in heating, cooling or both. That usually tells us which part of the system to focus on before we arrive.
