Mitsubishi Electric is one of the most common brands we see in homes and offices across the South East. When something goes wrong, the unit usually tells you roughly where. The trick is knowing which kind of unit you have, because a domestic wall unit and a commercial unit with a wired controller report faults differently.
The tables below are taken from Mitsubishi Electric’s UK service manuals for the MSZ-AP wall-mounted indoor units and the PKA wall-mounted Mr Slim indoor units. Codes and patterns can differ on other models, so treat this as a guide and check the manual for your exact model number.
M Series wall units: counting the blinks
On M Series wall units such as the MSZ-AP, faults show as blinks of the upper lamp of the OPERATION INDICATOR. Count the blinks between pauses. The MSZ-AP service manual gives these patterns:
| Upper lamp blinks | Abnormal point | What the manual says |
|---|---|---|
| Even blinking at 0.5 second intervals | Miswiring or serial signal | No serial signal from the outdoor unit for 6 minutes |
| 2 | Indoor coil or room thermistor | Short or open circuit |
| 3 | Indoor fan motor | No rotation feedback while the fan should be running |
| 4 | Indoor control system | Memory can’t be read; indoor board replacement |
| 5 | Outdoor power system | Compressor stopped 3 times within a minute of starting (overcurrent or start-up failure) |
| 6 | Outdoor thermistors | Short or open circuit during compressor operation |
| 7 | Outdoor control system | Outdoor board memory read failure |
| 14 or more | Other outdoor abnormality | Engineer checks valves and uses outdoor failure recall |
Source: Mitsubishi Electric MSZ-AP service manual. On multi-split systems, the manual also describes the upper lamp lit with the lower lamp blinking: the indoor units are set to conflicting modes, such as one on COOL and another on HEAT. That one you can fix yourself by setting them all to the same mode.
The same manual includes a separate failure-recall table that engineers use with the remote, where the blink counts mean different things. That’s why a blink count on its own is a starting point, not a diagnosis.
Mr Slim and wired controllers: check codes
Commercial and Mr Slim indoor units with a wired controller show a check code on the controller screen. These are from the PKA-M KA2 service manual:
| Code | Meaning (Mitsubishi Electric) |
|---|---|
| P1 | Room temperature thermistor (TH1) short or open |
| P2 | Liquid pipe temperature thermistor (TH2) short or open |
| P4 | Drain float switch contact failure or disconnected |
| P5 | Drain overflow protection, or drain pump fault |
| P6 | Icing or overheating protection operating |
| P8 | Pipe temperature out of the expected range compared with room temperature |
| P9 | Condenser/evaporator pipe thermistor (TH5) short or open |
| PA | Forced compressor stop due to a water leakage abnormality |
| Pb | Indoor unit fan motor error |
| PL | Abnormal refrigerant circuit |
| E0, E3, E4, E5 | Signal transmission or receiving errors between the remote controller and the indoor unit |
| E1, E2 | Remote controller board error (memory or clock) |
| E6, E7 | Indoor/outdoor unit communication error |
| Fb | Indoor unit control system error |
| EE | Indoor and outdoor unit combination error |
U-series codes come from the outdoor unit, and the manual refers engineers to the outdoor unit’s own service manual for them. If you see a U code, note it and call an engineer.
What the common codes usually lead to
- Thermistor codes (2 blinks, P1, P2, P9). Usually a sensor or its connector. A modest repair once confirmed by testing.
- Fan codes (3 blinks, Pb). Fan motor, connector or board. Don’t keep running a unit with an indoor fan fault, as the coil can ice up.
- Drain codes (P4, P5, PA). Often a blocked condensate line or a failed pump. See /air-conditioning-condensate-pumps/.
- Communication codes (even blinking, E6, E7). Wiring between units, a supply problem or a board. Common after building work or storms.
- Outdoor power and refrigerant codes (5 blinks, 14+ blinks, PL, U codes). Compressor, valves or refrigerant circuit. Always an engineer job.
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What you can safely check yourself
- Mode clash on multi-splits. Set all indoor units to the same mode.
- Filters. Clean them if they’re dusty. A clogged filter can trigger icing protection (P6).
- Power. Check the isolator and circuit breaker. Don’t reset a breaker that trips again.
- One restart. Switch off at the isolator for a few minutes and restart. If the same pattern returns, leave it off.
- Remote batteries if the unit doesn’t respond at all. Fresh batteries rule out the simplest cause.
- Water. If water is coming from the indoor unit, switch off and see /air-conditioning-leaking-water-causes-fixes/.
Leave panels, wiring and pipework alone. Mitsubishi Electric’s manuals are written for trained engineers working with the power isolated.
When to call an F-Gas engineer
Call us for any outdoor, refrigerant or communication fault that returns after one restart, and for any drain fault you can’t trace to an obvious blockage. Work on the refrigerant circuit must be done by an F-gas certified company using qualified engineers (GOV.UK). We’re F-Gas certified and NICEIC approved, so we can deal with both the refrigeration and the electrical side.
A repair call-out with diagnosis starts from £75 + VAT, and you get a fixed price before the repair goes ahead. We’re independent engineers rather than a Mitsubishi Electric accredited installer, and we work on the brand’s M Series, Mr Slim and City Multi systems. If your system is under warranty, tell us before we attend, so we can check what the terms require. See /mitsubishi-electric-air-conditioning/ and /air-conditioning-repair/.
Preventing repeat faults
Many of the codes above, especially drain, icing and sensor faults, are less likely on a system that’s serviced every year. An annual service cleans filters and coils, flushes the condensate drain and checks electrical connections and refrigerant performance. See /air-conditioning-annual-service-what-is-included/. Servicing costs £95 to £150 + VAT per year with us.
Quick answers
Why is my Mitsubishi Electric air conditioner flashing?
A blinking upper lamp on the OPERATION INDICATOR is a fault signal. Count the blinks and compare with the table. On the MSZ-AP, for example, 2 blinks points to a thermistor and 3 blinks to the indoor fan motor.
What does P8 mean on Mitsubishi Electric air conditioning?
The PKA service manual lists P8 as a pipe temperature error: the pipe temperature is outside the expected range compared with the room. It often needs an engineer to check the refrigerant circuit and sensors.
What does E6 mean on a Mitsubishi Electric controller?
E6 and E7 are communication errors between the indoor and outdoor units. Check the power, try one restart and call an engineer if it returns.
Can I reset a Mitsubishi Electric fault code?
You can switch off at the isolator and restart once. If the same code or blink pattern comes back, leave the system off and call an engineer, as repeated resets can cause further damage.
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This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.