Aircon Shutdown, Thermistor Drift

Case Details

What We Checked

• Compressor ran steadily for the first hour with consistent cooling output.
• After roughly ninety minutes, the compressor stopped even though the room felt noticeably warmer than setpoint.
• Room thermistor resistance measured several degrees below the actual room temperature when compared to the manufacturer spec table.

The Diagnosis

The room thermistor had drifted with age. After seven years of continuous use, the sensor's resistance characteristics had shifted — its output told the PCB the room was at setpoint when the actual temperature was still several degrees higher. The board shut the compressor down exactly as designed, following the data it received. It was not malfunctioning. The previous PCB replacement had no effect because the new board received the same drifted readings from the same worn sensor. The shutdown pattern was consistent rather than erratic, which is characteristic of drift rather than a failing component — a failing thermistor produces irregular readings, while a drifted one produces a consistent offset.

What Fixed It

We fitted a matched replacement room thermistor — sourced to the correct resistance curve for this Mitsubishi Electric model. After installation, we ran a full cooling cycle and monitored the compressor behaviour. The compressor stayed on continuously until the room genuinely reached the set temperature, then cycled normally. We also tested the coil thermistor to confirm it had not drifted by the same margin. Both the new PCB from the previous contractor and the replacement thermistor were now working correctly together. No further part replacements were needed.

The unit ran through a full afternoon without cutting out. Both the new PCB and the original one it replaced were functioning correctly all along.

Why This Happens

Why replacing the PCB does not fix a sensor problem.

• A clean shutdown with no error code usually means the PCB believes it has done its job — the room has reached setpoint. The board is following its logic correctly. The question is whether the temperature data it receives from its sensors is accurate.
• Thermistors are resistors whose electrical resistance changes predictably with temperature. Over years of continuous use, the material properties drift slightly. A reading even two degrees below actual room temperature is enough for the board to conclude that setpoint has been reached and shut the compressor down early.
• Testing thermistor resistance against the manufacturer specification table takes a few minutes with a multimeter. It should be the first diagnostic step for intermittent shutdowns because it rules out the cheapest possible cause before more expensive parts are considered.
• Ask your technician whether they tested the sensor readings before recommending a PCB replacement. A board swap costs significantly more than a thermistor, and if the sensor is the real cause, the new board will produce the same shutdown pattern.