Aircon Auto Mode Shutting Off, Faulty Sensor

Case Details

What We Checked

• Room thermometer read 26°C at panel height.
• Unit display showed 19°C — a 7-degree gap from actual room temperature.
• Sensor output was unstable, jumping between 18°C and 25°C over a few seconds.
• Outdoor ambient sensor read steady and within expected range.

The Diagnosis

The indoor ambient thermistor had degraded internally. Its resistance was drifting unpredictably instead of tracking room temperature steadily. When the reading dipped to 19°C, auto mode logic concluded the room had reached the set point and shut down the compressor. Seconds later, the resistance shifted again, the reading climbed back toward 25°C, and auto mode restarted cooling. This rapid on-off cycle repeated because the sensor output never stabilised long enough for the control board to hold a consistent operating state. The outdoor sensor and control board logic were both functioning correctly. The fault was isolated to the single thermistor component on the indoor unit.

What Fixed It

We replaced the degraded ambient thermistor with the correct rated part for this Daikin model. After installation, we confirmed that the panel display matched the independent thermometer reading within one degree at multiple room positions. We then ran the unit through a full auto-mode cycle at the client's usual set temperature. The compressor ramped down smoothly when the room reached the target, held steady without cycling, and restarted only when the room temperature naturally rose. No PCB replacement or firmware reset was needed.

Auto mode held a steady cycle without unexpected shutdowns. The panel temperature matched room conditions and the client returned to using auto mode daily.

Why This Happens

When auto mode misbehaves, check the temperature input first.

• Auto mode decisions depend entirely on what the sensor reports. If the sensor drifts, the unit follows instructions perfectly but acts on wrong data.
• Comparing the panel readout against an independent thermometer takes under a minute. A persistent gap points to the sensor, not the logic.
• Sensor replacement costs a fraction of a PCB swap. Ruling it out first avoids unnecessary board-level diagnosis.
• A degrading thermistor often starts with intermittent errors before failing consistently. If auto mode works some days but not others, the sensor may be drifting rather than dead.