- HVAC
- Air conditioner
P9
Mitsubishi Electric P9 on AC: indoor TH5 pipe sensor fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
P9 on a Mitsubishi Electric split unit means the indoor condenser/evaporator thermistor (TH5) stayed shorted (90 °C / 194 °F or higher) or open (-40 °C / -40 °F or lower) past the 3-minute resume protection window. Switch the power off and back on once; if P9 returns, book a technician.
Safety first
Stop and check this first
- If P9 returns after a single power reset, do not keep cycling the breaker because an actual refrigerant fault could be driving the coil to extreme temperatures.
- Never open the indoor unit or pull board connectors with power on: manual OCH639 warns that high voltage is applied to the indoor controller board.
Safe checks you can perform
- Confirm check code P9 on the controller or blink pattern
- Check whether outdoor conditions fall in the bypass window
- Single power reset at the breaker
- Inspect connector CN44 and measure TH5 resistance
Leave these tasks to a qualified professional
- Do not attempt this professional task: Re-seat connector CN44 and test thermistor TH5
- Do not attempt this professional task: Replace the indoor controller board only if readings diverge
- Do not attempt this professional task: Compare coil temperature in test run mode using PAC-SK52ST
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.
Re-seat connector CN44 and test thermistor TH5
What they doThe technician checks connector CN44 on the indoor controller board for contact failure, inspects the TH5 wiring harness, and measures the resistance of condenser/evaporator thermistor TH5 against the OCH639 chart.
They must show youconnector CN44 on the indoor board and the multimeter ohm reading for thermistor TH5.
In writingcode P9, the measured TH5 resistance in ohms, whether CN44 was re-inserted or TH5 replaced, and the date.
Replace the indoor controller board only if readings diverge
What they doIf TH5, its wiring, connector CN44, and the refrigerant circuit are sound but test run mode shows an extreme difference between actual and reported condenser/evaporator pipe temperature, the technician replaces the indoor controller board.
They must show youthe temperature discrepancy proving the board input is faulty and the replaced indoor board.
In writingthe part number of the replacement indoor controller board, the prior TH5 and CN44 test results, and the date.
Compare coil temperature in test run mode using PAC-SK52ST
What they doThe technician runs the unit in test run mode and connects the A-control service tool (PAC-SK52ST) to the outdoor controller board to compare the reported condenser/evaporator pipe temperature against the actual pipe temperature.
They must show youthe pipe temperature read via the controller board alongside the actual thermometer reading on the pipe.
In writingboth temperature readings during the test run, the finding on the refrigerant circuit or indoor board, and the date.
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.
Confirm check code P9 on the controller or blink pattern
- Who:
- you.
- How:
- check whether the wired remote displays P9 or, when running self-diagnosis from the wireless IR remote (press CHECK twice), the indoor operation indicator sounds 2 beeps and flashes 2 times in pattern A.
- Result that matters:
- confirming that the stored indoor fault specifically identifies the TH5 pipe sensor (P9).
- If P9 or the 2-blink pattern is present:
- note whether the unit was cooling, drying, or heating when it stopped.
- If a different blink count or check code appears:
- follow the guide for that specific code.
- Evidence to keep:
- a clear photo of P9 on the wired controller or a short clip of the 2-blink pattern.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Check whether outdoor conditions fall in the bypass window
- Who:
- you.
- How:
- note whether the stop happened during cooling or within the first 16 minutes of cooling start-up when the outdoor temperature was below -22 °F (-30 °C), where manual OCH639 states P9 detection is bypassed.
- Result that matters:
- verifying that P9 was triggered during active monitoring in cooling, drying, or heating.
- If the fault appeared during normal operating conditions:
- proceed with a single power reset.
- If the outdoor ambient was extremely cold during a cooling attempt:
- switch off and wait for normal ambient conditions.
- Evidence to keep:
- the operating mode and approximate outdoor temperature when P9 occurred.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Single power reset at the breaker
- Who:
- you, at the main breaker.
- How:
- turn the air conditioner off, switch the main power off and back on once as instructed in manual OCH639, and wait past the 3-minute resume protection period to see if normal operation resumes.
- Result that matters:
- clearing P9 if it was triggered by a one-off transient reading.
- If the unit runs normally past 3 minutes without showing P9:
- keep monitoring normal operation.
- If the unit stops again after 3 minutes with P9:
- leave it off and arrange a service visit.
- Evidence to keep:
- the time of the power reset and how many minutes passed before P9 returned.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Inspect connector CN44 and measure TH5 resistance
- Who:
- technician for P9.
- How:
- with main power isolated, access the indoor controller board, check connector CN44 for insertion failure, inspect the TH5 lead wire for breaks, and measure TH5 resistance with a multimeter.
- Result that matters:
- ruling out a loose CN44 plug or an open/shorted TH5 thermistor before suspecting the refrigerant circuit or PCB.
- If CN44 is loose or TH5 resistance is out of spec:
- re-seat CN44 or replace thermistor TH5, then restore power and check restart.
- If CN44 and TH5 resistance are normal:
- move on to test run pipe temperature checks.
- Evidence to keep:
- the ohm reading of TH5, the temperature at measurement, and the condition of connector CN44.
High voltage is present on the indoor controller board and fan connector CNMF: isolate mains power before opening the indoor unit.
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
- error P9 Mitsubishi Electric
- Mitsubishi Electric error P9
- Mitsubishi Electric P9
- P 9
- P9
- TH5 P9
What it means
According to Mitsubishi Electric service manual OCH639 (PKA-A24/30/36KA7 indoor units), check code P9 points to the indoor condenser/evaporator temperature thermistor (TH5), also signaled by 2 blinks and beeps under Output pattern A. When an open circuit (-40 °F [-40 °C] or less) or short circuit (194 °F [90 °C] or more) is detected during cooling, drying, or heating (except defrosting), the unit enters 3-minute resume protection and flags P9 if it does not recover within 3 minutes. Code P9 alone never proves that TH5 or the board has failed.
Warnings and stop conditions
- Mediumeditorial warning
If P9 returns after a single power reset, do not keep cycling the breaker because an actual refrigerant fault could be driving the coil to extreme temperatures.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- Highsource verified
Never open the indoor unit or pull board connectors with power on: manual OCH639 warns that high voltage is applied to the indoor controller board.
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
- Poor contact at connector CN44 on the indoor boardCommon
Connector CN44 for thermistor TH5 on the indoor controller board has an insertion failure or loose pin contact.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- Defective TH5 thermistor resistance characteristicsCommon
The condenser/evaporator thermistor TH5 has drifted out of specification and reports a resistance matching 194 °F (90 °C) or above, or -40 °F (-40 °C) or below.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- Broken wire or intermittent contact in TH5 harnessPossible
The lead wire running from thermistor TH5 to connector CN44 has an internal break or intermittent connection.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- Extreme coil temperature from a refrigerant circuit faultPossible
A defective refrigerant circuit pushes the actual condenser/evaporator pipe temperature to 194 °F (90 °C) or higher, or -40 °F (-40 °C) or lower.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
- Defective indoor controller circuit boardLess common
The sensing circuit on the indoor controller board reads an extreme difference compared with the actual condenser/evaporator pipe temperature.
Evidence: Mitsubishi Electric split-system heat pump service manual OCH639, indoor units PKA-A24KA7, PKA-A30KA7, PKA-A36KA7
Virtual technician
Does P9 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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