Safety: Disconnect power. Sharp metal edges around the coil can cut; wear gloves.

Procedure:

Access evaporator area: Remove front cover and any internal shrouds to expose the evaporator coil.
Identify defrost sensor: Some Mitsubishi Electric models use a dedicated defrost thermistor clipped to the lower part of the evaporator or suction line, separate from the main coil sensor.
Check mounting: Ensure the sensor is firmly clipped to the coil/tube. If it has fallen away, reattach using the original clip or a thermal clip.
Inspect wiring: Look for damaged insulation, pulled wires, or loose connectors between the sensor and PCB.
Measure resistance: With a multimeter, check the thermistor resistance at room temperature and compare to spec (NTC curve). If reading is open or shorted, replace the sensor.
PCB check: If the sensor and wiring are good, the defrost circuit on the main PCB may be faulty and require replacement.

Note: Do not bend or kink refrigerant tubing while accessing the sensor.

Unit displays E4 and stops; compressor does not run.

Possible Causes