Aircon shuts off by itself traced to drifted thermistor, not PCB
Aircon case in Siglap, Singapore: electrical/control traced to room temperature thermistor resistance had drifted outside normal range, sending incorrect readings to the PCB and causing premature shutdown after targeted diagnosis checks.
Case details
What client reported
The master bedroom aircon runs for a while then switches off on its own. It restarts after some time but keeps repeating the same cycle. Another company checked and said the indoor PCB was faulty and needed replacing. The unit is ten years old and the client was weighing whether to replace the whole unit instead.
What we found
A unit that shuts off at predictable intervals points to a sensor issue rather than a board failure. We tested the thermistor reading before inspecting the PCB.
- Room temperature thermistor was reporting a reading that did not match the actual room temperature
- The thermistor resistance had drifted outside the normal range for its rated curve
- PCB was responding correctly to the data it received — it shut the compressor down because the sensor told it the room had reached temperature
- After replacing the thermistor, the board received an accurate reading and the unit ran continuously without shutting off early
The room temperature thermistor had aged and its resistance had drifted. It was reporting a lower temperature than the actual room conditions. The PCB read this as the room having already reached the set temperature and shut the compressor down. When the sensor reading drifted back up, the compressor restarted. This created the repeating on-off cycle.
What we did
GOOD NEWS — the PCB was working correctly. The room temperature thermistor had drifted out of range and was sending the wrong reading to the board. Replacing the thermistor restored accurate temperature sensing. No PCB replacement was needed.
After the thermistor was replaced, the unit ran a full cooling cycle and held the set temperature without shutting off early. The PCB continued to function normally with the correct sensor input. No board replacement was needed.
Timeline
Day 1
Unit keeps shutting off — told the indoor PCB is faulty
Day 1
Tested thermistor resistance against actual room temperature before condemning the PCB
Day 1
Drifted thermistor replaced — unit ran full cycle without shutdown
What we learned
Thermistor drift vs PCB failure — how shutdown patterns differ.
- A drifted thermistor sends a reading that is consistently off by a fixed amount. The PCB reads this as the room being at the wrong temperature and shuts the compressor down too early or too late. The shutdown pattern is predictable and repeatable.
- Replacing a thermistor is a minor repair compared to a full PCB swap. Testing the sensor first rules out the cheaper fix before committing to the more expensive one.
- A failing PCB produces erratic behaviour — random shutdowns, unresponsive controls, or error codes that change each time. The pattern is inconsistent. If the shutdown follows the same pattern every time, the sensor is the more likely cause.
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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