Why Does My Aircon Fan Keep Running After Turning Off?
A short fan run after shutdown is usually a built-in drying cycle to prevent mould on the coil. But if the fan keeps running far longer than usual, or stops unpredictably, the control path is likely at fault.
1. Normal Post-Off Drying Cycle
How This Works
The evaporator coil is covered in condensate moisture during a cooling cycle. When the compressor stops, that moisture sits on a warm coil in humid air, ideal for mould and bacteria. Most manufacturers program a short post-off fan delay to dry the coil before the unit goes dormant. This is a design feature, not a fault.
How To Tell
A drying cycle terminates cleanly every time and is consistently short. Unlike a relay fault, the fan does not run indefinitely or require power cycling to stop. Unlike electrical instability, there are no unusual sounds or odours. If the unit has an explicit auto-dry or self-clean mode, the run-on is designed behaviour.
- Fan stops on its own after a short period.
- No unusual smell or noise.
- Next startup is normal.
How We'd Confirm It
We confirm the run-on duration matches the model's built-in drying cycle before treating the pattern as a repair case.
Do not approve a board replacement based on a short post-off run. Check the unit's settings for an auto-dry or self-clean mode first. The behaviour is usually documented in the manual.
2. Sticky Fan Relay Or Stop-Command Path Fault
How This Works
The fan relay on the indoor PCB receives a stop signal when the unit gets an off command. If that relay contact is sticky from carbon buildup or contact welding, it may hold the fan circuit closed after the stop signal is sent. The board believes the fan has stopped; the relay has not released.
How To Tell
The fan runs far longer than a normal drying cycle, or stops and restarts at random. Unlike a drying delay, the run does not terminate cleanly. Unlike electrical instability, there are no odours or breaker events. If the off command response is also inconsistent across attempts, the command path is faulty rather than the motor.
- Fan run-on feels much longer than usual.
- Remote off command response is inconsistent.
- Pattern repeats across different attempts.
How We'd Confirm It
We test relay switching response and board output signals to confirm whether the relay is stuck or the board is not sending the stop command.
Do not replace the fan motor before relay switching response is tested. The relay is the cheaper first check, and a sticky contact produces the same surface symptom as a failing motor.
3. Unstable Mains Disrupting Stop Sequence
How This Works
Unstable mains power, including voltage fluctuations from shared distribution in older HDB blocks, can disrupt the PCB's logic supply. If the logic rail drops momentarily during an off-command sequence, the board may execute the command partially: the compressor stops but the fan relay does not receive its stop pulse. The fan continues running because the command was never fully processed.
How To Tell
Physical symptoms alongside the fan run-on are the distinguishing markers: a burning odour near the indoor unit, a breaker trip with operational changes, or the fan running at irregular speeds. Unlike a normal drying delay, the run is not predictable. Unlike a pure relay fault, the issue involves electrical stress on the motor circuit.
- Burning or electrical odour appears.
- Breaker trip appears with operation changes.
- Fan behaviour is erratic with power events.
How We'd Confirm It
Stop repeated on-off testing. We check voltage stability at the board and inspect relay contacts for arcing or heat damage.
Stop using the unit if you smell burning or hear unusual motor sounds. Turn the isolator off and do not attempt further on-off cycling until the electrical fault is assessed.
Related Reading
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