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Outdoor Unit Not Running
The indoor head blows air and the lights are on, but the box outside is silent and its fan never turns.
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What's happening?
The outdoor unit — the condenser — holds the compressor, the fan and, on an inverter system, the drive electronics. It is the half that actually moves the heat, so an indoor head running on its own produces nothing but circulated room air. A silent condenser usually means one of three things: it is not getting a supply, it is getting a supply but a component in it has failed, or it is being told not to run. That last one catches people out, because several normal behaviours look like a dead unit. Inverter systems idle at very low speed and can be almost inaudible, and every system enforces a compressor restart delay of a few minutes after it stops, so a unit that has just cycled off will sit silent for a while by design.
What usually causes it?
- Isolator switch beside the outdoor unit knocked or turned off, often during garden or building work
- Circuit breaker or fuse for the air conditioning circuit tripped at the consumer unit
- Fan capacitor failed, so the fan hums or sits still and the unit trips on high pressure
- Contactor or relay burnt out, so the compressor never receives its supply
- Control signal not reaching the outdoor unit because the interconnecting cable is damaged or corroded
- Compressor tripped on its internal thermal overload after overheating
- Unit still inside its normal restart delay, or holding off because the room has reached setpoint
Is it dangerous?
The unit standing still is not dangerous. The way people investigate it can be. The outdoor enclosure contains mains terminals and, on inverter units, capacitors that hold a charge after the supply is removed, so the lid stays on. If the breaker trips again the moment you reset it, leave it off and stop — that is a fault finding a path to earth, not a nuisance trip. Anything scorched, melted or smelling hot at the isolator means isolate and call, not try again.
Can I fix it myself?
Check the obvious supply points, which are genuinely often the answer. Find the isolator on the wall near the outdoor unit and confirm it is on. Check the consumer unit for a tripped breaker or RCD on the air conditioning circuit and reset it once. Then wait: turn the system off at the remote, leave it a full five minutes, and turn it back on so any restart delay has expired. Clear anything piled against the unit. Do not remove the outdoor casing or the fan grille.
When should you call an air conditioning engineer?
Call an air conditioning engineer if the isolator and breaker are on and the unit still never starts, if the breaker trips again after a reset, or if the fan hums without turning — a classic failed capacitor. Because this fault sits across the electrical supply and the refrigerant machine, an air conditioning engineer is the right first call; they will refer the fixed wiring back to an electrician if the fault turns out to be upstream of the isolator.
What will the air conditioning engineer actually do?
- Prove the supply at the outdoor isolator and check the circuit protection back at the board
- Test the fan capacitor, the contactor and the compressor windings
- Check the interconnecting cable and control signal between indoor and outdoor units
- Read the fault log on the outdoor board for a stored trip that has since cleared
- Replace the failed capacitor, contactor, fan motor or board and recommission
- Measure running current and pressures once it starts, to confirm nothing else has been damaged
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