- HVAC
- Air conditioner
P1
Mitsubishi Electric P1 on an air conditioner: room temperature sensor fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Mr. Slim PKA wall units, P1 means the intake temperature thermistor (TH1) reads an impossible value. Switch the power off and on again; if P1 returns, call the service technician without opening the unit.
Safety first
Stop and check this first
- Mitsubishi Electric requires all supply circuits to be disconnected before reaching the terminals. If refrigerant leaks, ventilate the room: in contact with a flame it releases toxic gases. Board, wiring and refrigerant circuit are technician work.
Safe checks you can perform
- Code on the remote and lamp blinks
- Restart from the circuit breaker
- Measuring the TH1 thermistor
Leave these tasks to a qualified professional
- Do not attempt this professional task: Operating test after the repair
- Do not attempt this professional task: Measure sensor and connector before replacing the board
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Operating test after the repair
What they doAfter the work, the technician runs the unit for more than 3 minutes and checks that P1 does not return.
They must show youthe unit running with no code.
In writingthe test date and result.
Measure sensor and connector before replacing the board
What they doThe technician measures TH1 with a multimeter, checks CN20 and the wiring, and only then considers the indoor board.
They must show youthe measured TH1 value against Mitsubishi's table.
In writingwhat was measured, which part was replaced and why.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Code on the remote and lamp blinks
- Who:
- you.
- How:
- read the code on the remote and count the blinks of the operation lamp on the indoor unit.
- Result that matters:
- that it is P1 (one blink). Mitsubishi classes it as a fault detected by the indoor unit itself.
- If you see P1 or one blink:
- the room sensor is the suspect; this code does not point at the outdoor unit.
- If you see another code:
- this page does not apply; look up that exact code.
- Evidence to keep:
- a photo of the remote with the code, with date and time.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Restart from the circuit breaker
- Who:
- you.
- How:
- switch the unit off, turn off its circuit at the consumer unit, wait a few minutes and restore power.
- Result that matters:
- whether P1 comes back once it runs again.
- If it does not return:
- it may have been a one-off reading; note the date in case it repeats.
- If it returns:
- the reading is still out of range; call the service technician and stop resetting.
- Evidence to keep:
- a note of how many times P1 appeared and in which mode the unit was running.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Measuring the TH1 thermistor
- Who:
- an air conditioning technician.
- How:
- with the unit isolated, unplugs TH1 from CN20 and measures its resistance with a multimeter.
- Result that matters:
- Mitsubishi gives 15.0 kΩ at 0 °C and 6.3 kΩ at 20 °C as reference.
- If the resistance does not match the temperature:
- the thermistor is confirmed as the cause.
- If it measures correctly and the connector is firm:
- the sensor is ruled out and the indoor board becomes the suspect.
- Evidence to keep:
- a job sheet with the measured value and the room temperature at the time.
Requires isolating the power supply and opening the indoor unit: technician only.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- HVAC
- Brand
- Mitsubishi Electric
- Product type
- Air conditioner
Known code variants
- Baxi heat pump P1
- Baxi HP40 P1
- Baxi HP50 P1
- Mitsubishi P1
- Mr. Slim P1
- PKA P1
What it means
Mitsubishi Electric links P1 to the indoor room temperature thermistor TH1. The control flags it when it reads a short (90 °C or more) or an open circuit (-40 °C or less) and the reading does not recover within 3 minutes. Indoor and outdoor units stop and the operation lamp blinks once. The code does not tell a drifted thermistor from a broken wire, the CN20 connector or the indoor board: only a technician's measurement separates them.
Warnings and stop conditions
- Highsource verified
Mitsubishi Electric requires all supply circuits to be disconnected before reaching the terminals. If refrigerant leaks, ventilate the room: in contact with a flame it releases toxic gases. Board, wiring and refrigerant circuit are technician work.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Symptoms and causes
Probable causes · in order
- TH1 thermistor out of characteristicCommon
The sensor is present and connected but its resistance does not match the real temperature. The technician sees this when measuring it against the maker's table.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- CN20 connector or sensor wiringPossible
A loose contact at CN20 or a broken wire gives an open reading even with a good sensor; it shows up when the sensor measures correctly off the circuit.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- Indoor controller boardPossible
If sensor and wiring measure correctly and the reading still differs from the real room temperature, Mitsubishi says to replace the indoor board.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Virtual technician
Does P1 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Mitsubishi Electric · 2016 · manual
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