If the indoor unit runs but the outdoor unit is not running, the system can’t heat or cool: you’ll get room-temperature air from the fan. Check the isolator switch beside the outdoor unit and the circuit breaker first. If both are on, the likely causes are a failed capacitor, a failed contactor, a fan motor fault, a protection lockout or a communication fault between the units. All of these need a technician; the outdoor unit contains live electrical parts and refrigerant under pressure.
Checks you can do safely
- Mode: the outdoor unit only runs in cool, heat or dry, not fan mode.
- Isolator: the weatherproof switch on the wall beside the outdoor unit must be on.
- Circuit breaker: some systems have a separate breaker for the outdoor unit. Reset it once only.
- Wait: many systems delay the outdoor unit for a few minutes after start-up to protect the compressor.
- Error code: check the indoor unit for a flashing lamp and note the pattern.
Causes and what each one needs
| Cause | What you’ll notice | Who fixes it |
|---|---|---|
| Isolator off or breaker tripped | Outdoor unit silent | You (once); technician if it trips again |
| Failed capacitor | Humming, but the fan and compressor don’t start | Technician |
| Failed contactor or relay | Silent, or clicks without starting | Technician |
| Outdoor fan motor fault | Compressor hums, fan still; unit overheats and stops | Technician |
| Communication fault between units | Error code; indoor fan runs | Technician |
| High-pressure or overheat protection | Runs, then stops on hot days; restarts later | Clear the coil; technician if it continues |
| Compressor failure | Fan runs, compressor never starts, no cooling | Technician; see compressor cost |
Do not remove the outdoor unit’s covers. Capacitors can hold a charge even with the power off.
Related guides
Book a technician, or go back to the troubleshooting hub.
For general advice on staying safe around electrical appliances at home, see Energy Safe Victoria.
Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles