Aircon Thermistor Fault, Not PCB

Case Details

What We Checked

• Room thermistor resistance measured against the rated curve — reading corresponded to a temperature several degrees below the actual room conditions.
• Shutdown interval was consistent across three consecutive cycles, matching a sensor-driven pattern.
• PCB responded correctly to a manual test input, confirming its logic and relay outputs were functioning.

The Diagnosis

The room thermistor had aged over ten years and its resistance had shifted from the original spec. A thermistor is a sensor whose resistance changes with temperature. The PCB uses that resistance value to work out room temperature. When the resistance drifts, the reading no longer matches what is real. In this case, the sensor told the PCB the room was several degrees cooler than it truly was. The PCB took that false value, decided the room had already reached setpoint, and shut the compressor down. As the room warmed past the false reading, the gap reopened and the compressor restarted — creating the repeating on-off cycle at steady intervals that the client described.

What Fixed It

We explained that the PCB was working correctly — it was shutting down the compressor because it was getting wrong data from the drifted sensor, not because the board itself was faulty. We replaced the thermistor with a matched sensor rated to the same curve specified by Mitsubishi Electric for this model. After fitting the new sensor, we ran a full cooling cycle and watched the compressor. It stayed on until the room truly reached setpoint, then cycled off normally. The new sensor's resistance reading matched the actual room temperature within the expected range.

The on-off cycling stopped. The unit held the set temperature through a full afternoon run with no early shutdown.

Why This Happens

How to tell a sensor fault from a board fault by the shutdown pattern.

• A drifted thermistor produces the same pattern every time. The unit shuts off at roughly the same interval because the gap between the real temperature and the false reading stays constant. The PCB is doing exactly what it is meant to do — just working from wrong data.
• A failing PCB produces erratic behaviour — random shutdowns at different intervals, changing error codes, or controls that sometimes respond and sometimes do not. That kind of random pattern is the hallmark of a board problem. It looks very different from the steady cycling of a sensor fault.
• When the shutdown pattern is steady and repeating, testing the sensor resistance against the maker's spec table should come first. Ask your technician to measure the thermistor with a multimeter and compare the reading to the rated curve. This takes minutes and costs nothing.
• Thermistors are wear parts that drift over years of use. A ten-year-old sensor running 8-12 hours daily has gone through thousands of heating and cooling cycles, so some drift is normal. Swapping the sensor before blaming the board can save hundreds of dollars.