Aircon Shutting Off Mid-cycle: Thermistor Drifted Out of Range
Aircon case in Clementi, Singapore: cooling loss traced to liquid pipe thermistor resistance drifted out of range after targeted diagnosis checks.
Case Details
| Unit | DaikinWall-mounted |
|---|---|
| Age | 10 years old |
| Location | CondoClementi, Singapore |
| Reported | The aircon starts cooling normally but shuts off after about ten to fifteen minutes every time, with an error code on the display. It has been happening more often over the past few weeks. |
What We Checked
- C4 error code was displayed after each shutdown.
- Unit started and cooled normally for the first ten to fifteen minutes before cutting out.
- Wiring between the thermistor and PCB was intact with no corrosion or loose connections.
- Thermistor resistance measured at room temperature was approximately 30 percent above the manufacturer specification.
- PCB was responding correctly to all other inputs and commands.
The Diagnosis
The liquid pipe thermistor — a small sensor clipped to the refrigerant pipe — had drifted out of its resistance range after years of thermal cycling. At room temperature, it read approximately 30 percent above the Daikin specification. This caused the PCB to receive inaccurate temperature readings, triggering a protective shutdown mid-cycle. The C4 error code flags specifically when the liquid pipe thermistor reading falls outside the expected range.
What Fixed It
We explained that the thermistor had degraded with age and was giving the PCB false temperature readings. The fix was straightforward — replace the thermistor sensor. The part costs around twenty-five dollars and takes about twenty minutes to swap. No PCB replacement, no refrigerant work, no major repair needed.
The thermistor was replaced, and the unit resumed normal operation with no mid-cycle shutdowns — the C4 error has not returned.
Why This Happens
Why thermistors drift and how to catch it.
- Thermistors are temperature-sensitive resistors that degrade over years of thermal cycling — their resistance shifts gradually out of specification.
- A drifted thermistor sends incorrect temperature readings to the PCB, which triggers protective shutdowns even when the system is operating normally.
- Measuring resistance at a known room temperature and comparing against the manufacturer specification is the definitive test — visual inspection reveals nothing.
Related Reading
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