Homeowner answer
Why is my outdoor AC unit not running?
An outdoor AC unit that stays off while the indoor fan blows has usually lost power at its breaker or has a failed capacitor, contactor or fan motor. Check that the thermostat is on Cool and the breaker is on. If the unit hums, clicks or still will not start, switch the system off and book a diagnosis.
Call soon
Without the outdoor unit nothing is cooling, and a unit that hums without starting can damage the compressor. Switch off and book soon.
Updated
When the indoor fan blows but the outdoor unit sits silent, the house gains heat with nothing removing it. The outdoor unit has its own power and its own parts, so the cause is usually a breaker, a setting or a failed electrical piece — in that order of likelihood.
What you can safely check
- Confirm the thermostat is calling for cooling
Do this:Set the mode to Cool, the fan to Auto, and the setpoint a few degrees below the room temperature. Then stand by the outdoor unit and wait.
You should see:A click or hum from the unit within a few minutes, as some systems pause on purpose before starting the compressor.
When to stop:Stop and move to the breaker if nothing happens at all after a reasonable wait.
- Check the outdoor unit's breaker
Do this:At the electrical panel, find the breaker labeled condenser or AC and reset it once by switching it fully off, then on. The outdoor unit sits on its own circuit, which is why the indoor fan can run without it.
You should see:The unit starting up within a few minutes, with the top fan spinning.
When to stop:Stop and leave the breaker off if it trips again — repeated resets stress the compressor, and the fault needs a technician.
- Look and listen at the unit
Do this:With the thermostat calling for cooling, note what the unit does: silent, humming without the fan turning, or clicking over and over. What you hear points straight at the part that failed.
You should see:A steady hum with the fan spinning is healthy; anything else is information for the technician.
When to stop:Stop and switch the system off at the thermostat if it hums without starting or clicks repeatedly — running it that way risks the compressor.
- Clear around the unit
Do this:Remove leaves, grass clippings and debris from around and on top of the cabinet, and trim plants back, without opening anything.
You should see:Open space on all sides and warm air rising steadily off the top while it runs.
When to stop:Stop at the grille — never reach into the fan or push it with anything, even with the system off. That only hides a failing motor and risks your hands.
- Allow the restart delay
Do this:After a trip, an outage or a shut-off, many systems wait several minutes before the compressor restarts, so leave the thermostat calling and give it time.
You should see:The compressor kicking in after its pause, and the air at the vents gradually cooling.
When to stop:Stop and book if the unit stays silent with the thermostat calling and the breaker confirmed — the remaining causes are electrical parts that need testing.
Likely causes
- Tripped breaker
The outdoor unit has its own circuit, so it can lose power while the indoor fan keeps running.
Handled by you
- Thermostat settings
If the thermostat is set to Fan or the setpoint is above room temperature, the outdoor unit stays off.
Handled by you
- Failed capacitor
The capacitor gives the fan and compressor motors the push they need to start. A weak one often causes a hum without starting.
Handled by a technician
- Failed contactor
The contactor is the switch that sends power to the outdoor unit. If it fails, the unit never gets the signal.
Handled by a technician
- Failed fan motor or overheated compressor
A worn motor, or a compressor that has shut itself off to cool down, keeps the unit from running.
Handled by a technician
Call a pro when…
Book a diagnosis if the breaker is on and the thermostat is calling for cooling but the unit stays off, hums without starting or trips the breaker again.
- The unit hums, buzzes or clicks but the fan will not spin — switch the system off first.
- The breaker trips a second time after one reset — leave it off.
- The unit stays silent with the thermostat calling for cooling and the breaker on.
- The fan spins but the air at the vents never turns cool.
- A burning smell, smoke or sparks — switch off at the thermostat and the breaker, and leave the home if there is smoke.
Next step
Book a diagnosis
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Common questions
Why is my indoor AC running but not the outside unit?
The indoor blower and outdoor unit are powered separately, so the blower can run while the outdoor unit has lost power or has a failed part. Check the breaker first, then book service.
Can I push the outdoor fan with a stick to start it?
No. Reaching into the fan or pushing it is dangerous, and it only hides a failing capacitor or motor. Switch the system off and call a technician.
Why does my outdoor unit hum but the fan will not spin?
Humming usually means power is reaching the unit but a motor cannot start, often because of a weak capacitor. Switch the system off to protect the compressor and book a diagnosis.