Aircon error code traced to drifted thermistor, not PCB board
Aircon case in HarbourFront, Singapore: electrical/control traced to thermistor sensor had drifted out of its operating range, sending inaccurate temperature readings to the PCB and triggering a protective shutdown after targeted diagnosis checks.
Case details
What client reported
The living room aircon keeps showing an error code and shutting itself off after running for a short while. It restarts sometimes but shuts down again. Another company diagnosed a PCB board failure and quoted for a replacement board. The unit is only four years old.
What we found
An error code with intermittent shutdowns on a relatively new unit suggested a sensor issue rather than a board failure. We checked the thermistor readings before testing the board.
- Error code corresponded to a temperature sensing fault — the board was detecting an out-of-range temperature reading
- Thermistor resistance was checked and found to be drifting well outside the manufacturer specification for the ambient temperature
- PCB board responded normally to all other inputs and commands — the board was not faulty
- After replacing the thermistor with a new sensor, the error code cleared and the unit ran continuously without shutting down
The thermistor sensor had degraded over time and its resistance was drifting outside the expected range. The PCB was receiving temperature readings that looked dangerously abnormal, so it triggered a protective shutdown. Each time the unit restarted, the drifted reading eventually triggered the shutdown again. The board was functioning exactly as designed — it was the sensor feeding it bad data.
What we did
Replacing the thermistor resolved the error immediately. The PCB board did not need replacing. The client saved the cost of a board replacement by addressing the actual faulty component — a small sensor that costs a fraction of a PCB.
The error code was cleared and the unit resumed continuous operation without shutdowns. The original PCB board remains in the unit. Only the thermistor was replaced.
Timeline
Day 1
LG unit showing error code and shutting down intermittently — quoted for PCB replacement
Day 1
Checked thermistor resistance against the manufacturer spec before condemning the PCB board
Day 1
Thermistor replaced — error code cleared and unit ran continuously
What we learned
Error codes — when the sensor fails but the board gets blamed.
- A thermistor is a small temperature sensor that sends readings to the PCB. If its resistance drifts out of range, the board receives readings that look abnormal — like an extremely high or low temperature — and shuts the unit down as a safety measure.
- The PCB is doing its job correctly when it shuts down in response to a bad sensor reading. Replacing the board does not fix the problem if the faulty signal is still coming from the thermistor.
- Checking thermistor resistance against the manufacturer specification is a quick diagnostic step. If the reading is out of range, replacing the sensor is a fraction of the cost of a board replacement.
Best next step
If your unit is behaving similarly, start with the service path that fits this case before approving broader scope.
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