Mitsubishi Electric P8 Error: Heat Exchanger Temp Abnormality
Fix Mitsubishi Electric P8 error code (pipe thermistor temperature fault).
Heat exchanger pipe thermistor temperature abnormality (overheating or coil freeze-up protection triggered).
Probable Causes
Clogged indoor air filters choking airflow across evaporator coil.
Pipe thermistor out of resistance specification or unclipped from copper sensor well.
Dirty blower wheel or failing fan motor capacitor.
Diagnostic & Testing Steps
- •Indoor unit halts with operation lamp blinking 8 consecutive times.
- •Error code P8 shown on PAR-40 / PAR-33 wall controller.
- •Poor thermal capacity preceding safety shutdown.
Tools & Equipment Required
- ✓Digital Multimeter (Resistance Mode)Measure Linear Expansion Valve (LEV) stepper coil resistance (46 Ω ± 3 Ω on each pin).
- ✓Phillips #2 Screwdriver & 10mm Nut DriverRemove outdoor unit access panel and sound insulation blanket.
- ✓Strong Neodymium Testing MagnetManually cycle and exercise stuck LEV internal valve needle to clear mechanical binding.
- ✓Small Inspection FlashlightCheck LEV solenoid harness plug for corrosion or backed-out connector pins.
Fast Reset Sequence: HVAC Microprocessor & Fault Memory Clear
Before unmounting panels or beginning diagnostic procedures, attempt this manufacturer reset to clear transient microcomputer lockouts:
- 1.Isolate electrical power to both indoor wall units and outdoor compressor at the main breaker panel.
- 2.Leave powered down for 10 minutes to allow inverter board microprocessors to dump error registers.
- 3.On remote control, press and hold Cancel or Test button for 5 seconds until the display resets to normal temperature mode.
- 4.Restore power to outdoor unit first, wait 3 minutes for crankcase heater pre-warm, then switch on indoor units.
- 1
Step 1: Wash Air Filters and Clean Coil Fins
[ ] DoneOpen front grille, remove dust screens, wash in warm water, and gently vacuum aluminum coil surface.
Verification:Filters are clean and dry, permitting unobstructed airflow across coil. - 2
Step 2: Test Pipe Thermistor Ohmic Resistance
[ ] DoneDisconnect pipe thermistor from indoor control board and test resistance using digital multimeter.
Verification:Sensor reads approximately 10 kΩ at 77°F (25°C) and is securely seated in copper well. - 3
Step 3: Reset Controller and Run High-Speed Fan Test
[ ] DoneReassemble panels, restore power, and run unit in fan-only mode on high speed for 10 minutes.
Verification:Airflow is strong and uniform, and error P8 does not reappear on controller.
Repair Solutions
Wash mesh air filters with warm water and vacuum coil fins.
Measure pipe thermistor resistance with ohmmeter (standard 10 kΩ at 77°F / 25°C).
Check blower wheel freedom of rotation and confirm unblocked supply/return vents.
Frequently Asked Questions
Is error P8 dangerous to the Mitsubishi system?
P8 is a protective safety alarm preventing coil freeze-up or compressor strain before hardware damage occurs.
How often should filters be cleaned to prevent P8?
Cleaning filters monthly during peak summer and winter operation effectively prevents P8 occurrences.