Intermittent shutdown traced to drifted thermistor, not PCB fault
Aircon case in Cashew, Singapore: electrical/control traced to room temperature thermistor had drifted, sending a lower-than-actual reading to the PCB — causing the unit to think setpoint was reached and shutting off the compressor prematurely after targeted diagnosis checks.
Case details
What client reported
The aircon in our bedroom runs for two or three hours and then just stops. No blinking lights, no error code — it just goes quiet. Another company replaced the PCB board but it still does the same thing. They want to check the compressor next.
What we found
When a unit shuts off cleanly without an error code, the PCB usually thinks it has reached setpoint. We checked what the PCB was receiving from the sensors before suspecting the board or compressor.
- Unit ran normally for the first hour — cooling output was consistent and the compressor sounded healthy
- After about ninety minutes, the compressor stopped even though the room was noticeably warmer than the setpoint
- Room thermistor resistance measured against the manufacturer spec table — reading corresponded to a temperature several degrees below the actual room temperature
- The PCB was acting on the wrong reading and shutting down because it believed setpoint had been reached
The room temperature thermistor had drifted with age. Its resistance reading told the PCB the room was cooler than it actually was. When the gap between the false reading and the setpoint narrowed enough, the PCB shut off the compressor as designed. The PCB itself was working correctly — it was just receiving bad data.
What we did
We replaced the room thermistor with a matched replacement. The unit was tested through a full cooling cycle to confirm the compressor stayed on until the room actually reached setpoint.
The shutdowns stopped. The unit ran a full afternoon cycle without cutting out. The new PCB the previous contractor installed was fine — the original one likely was too. The fix was a sensor that costs a fraction of a PCB.
Timeline
Week 1
Another contractor replaced PCB — shutdowns continued
Week 3
Measured thermistor resistance at room temperature and compared to the manufacturer spec — reading was off by enough to cause early shutoff
Week 3
Thermistor replaced — unit ran full cycle without shutting down
What we learned
Intermittent shutdown: PCB vs thermistor.
- Thermistors drift over time. A room thermistor that reads two or three degrees low will cause the unit to think the room has reached setpoint and shut off the compressor early.
- Checking thermistor resistance against the spec table takes a few minutes and should be done before any PCB replacement is considered.
- A unit that shuts down without an error code is often misread as a PCB fault. But if the PCB is processing a wrong input, replacing it will not fix anything.
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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