New Board Didn't Fix It: Temperature Sensor Was The Problem All Along

• •Concern: A PCB replacement had already been paid for but did not resolve the issue. Previous advice was that the next step was a compressor check.
• •Key check: Measured thermistor resistance at room temperature and compared to the manufacturer spec. Reading was off by enough to cause early shutoff

A clean shutdown with no error code usually means the PCB believes it has done its job, the room reached target temperature. Rather than testing the board or compressor next, we checked what the board was actually seeing from its sensors. We ran the unit and monitored it through a full cycle. At intervals, we measured the room thermistor resistance with a multimeter and compared each reading to the manufacturer spec table using an independent thermometer.

• •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 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. Characteristic of drift rather than a failing component. A failing thermistor produces irregular readings; a drifted one produces a consistent offset.

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 compressor behaviour. The compressor stayed on until the room genuinely reached the set temperature, then cycled normally. We also tested the coil thermistor to confirm it had not drifted. Both the new PCB from the previous contractor and the replacement thermistor now worked 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 had been functioning correctly all along.

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 actual cause, the new board will produce the same shutdown pattern.