Why is my aircon outdoor unit not running?
Indoor unit on. Outdoor unit silent. The next checks are startup sound, protection signs, and whether the cooling call actually reaches the outdoor side before any compressor quote.
By Team Snowflake | Reviewed 30 May 2026
1. Command handoff or mode condition
The outdoor unit sits completely silent while the indoor fan runs and the remote responds normally. Often the outdoor unit is not faulty at all: the indoor unit is set to fan-only mode, the room has already reached the target temperature, or no cooling call is being sent. From where you stand, this looks identical to a dead compressor.
How to tell
This path has silence with normal indoor response. Unlike a start-circuit fault, there is no click, hum, or brief attempt. Unlike protection lockout, there are no breaker trips, flashing lights, or error codes. If settings are correct and silence remains, check the outdoor electrical path.
- Indoor fan runs and the display lights up, but no cool air arrives.
- The outdoor unit makes no sound at all, not even a click.
- It started after a mode, timer, or temperature change on the remote.
How we confirm it
We confirm the operating mode first. Then we check the set temperature against the room reading. We also check whether the indoor board is actually sending a cooling command. Only after that do we move to any electrical checks.
Avoid stopping at the remote settings before the outdoor electrical path is checked. Correct settings plus silence still needs electrical diagnosis.
2. Start circuit or outdoor electrical path fault
The outdoor unit receives a valid cooling call but cannot get moving. A failed run capacitor loses the charge needed to spin up the compressor motor. The compressor hums briefly, draws high current, then falls silent as protection trips. A pitted contactor makes intermittent contact and drops the circuit mid-startup. A relay that will not pull in breaks the circuit before the motor even attempts to turn.
How to tell
This path tries and fails. Unlike command handoff, the outdoor unit clicks, hums, or surges before cutting out. Unlike protection lockout, it does not announce itself with trips or codes. The first checks are capacitor, contactor, and relay, not compressor replacement.
- The indoor unit runs normally but cooling never starts.
- You hear clicking, a low hum, or a brief surge from the outdoor unit.
- The problem appeared suddenly rather than fading in over weeks.
How we confirm it
We test the run capacitor with a meter and inspect the contactor face for pitting. We also check the relay coil. These readings pinpoint which start component failed before we quote any part.
Avoid compressor or full replacement before capacitor, contactor, and relay readings are taken. These start parts are cheaper and commonly fail.
3. Protection lock or compressor-side fault
When overload, heat stress, or an unsafe electrical condition is detected, the system locks the outdoor unit off to prevent damage. Thermal overload cuts the compressor circuit when winding temperature crosses its threshold. The unit goes silent, cools over several minutes, then attempts to start again. In humid, poorly ventilated outdoor corridors common to HDB and older condo blocks, this cycle can repeat several times before anyone notices a pattern.
How to tell
This path cycles with protection signs. Unlike a start-circuit fault, the unit may run briefly, cut out, cool down, and try again. Unlike command handoff, the cooling call reaches the outdoor unit. Breaker trips, flashing codes, heat, or burning smell point to overload or unsafe operation.
- The breaker trips, an error code flashes, or starts fail again and again.
- The outdoor casing feels unusually hot after each attempt to run.
- A burning smell or unstable startup sounds appear alongside the no-start.
How we confirm it
We check compressor winding resistance and measure insulation to ground. We also read the protection circuit status. These readings tell us whether the lockout is recoverable or the compressor itself has failed.
Stop using the unit if it is tripping the breaker or giving off a burning smell, and do not force restarts. Each restart under winding stress accelerates insulation breakdown and can turn a recoverable fault into an irreparable one.
Related reading
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