Troubleshooting · Won't run

AC Outdoor Unit Not Running? Why the Fan or Compressor Won't Start

The indoor fan blows but the outside unit is silent, humming or its fan is still.

By Mark Curant·Updated October 14, 2026

Most likely causes

A tripped breaker or blown fuse in the outdoor disconnect
A failed capacitor (often humming with the fan still)
A failed or stuck contactor (silent, or clicks without starting)
A failed condenser fan motor (compressor runs, fan doesn't)
The compressor's overload protector has tripped from heat

Before you open anything

Turn the system off at the thermostat, then switch off power at the breaker and at the disconnect beside the outdoor unit. Never touch the capacitor, even with the power off: it can hold a dangerous charge. Refrigerant work is for licensed technicians only. Reset a tripped breaker once; if it trips again, leave it off.

Don't push the fan blades with a stick to get them started. It's a common tip online, but the fan can start suddenly. And don't touch the capacitor: it stores a dangerous charge even with the power off.

Check these first (free, 5 minutes)

Thermostat: set to Cool and below room temperature, so it's actually calling for cooling.
Breaker for the outdoor unit, and the disconnect box beside it.
Listen: is it silent, humming, or clicking? It narrows the cause down.
Look: is the fan on top spinning? Is the compressor (the round tank inside) running?

Causes, signs and repair costs

Tripped breaker or blown disconnect fuse

DIY or pro

Completely silent, breaker tripped

Typical cost: $0 to reset; service call if it trips again

Failed capacitor

Pro

Humming, fan not spinning or starting slowly; a bulging or leaking capacitor

Typical cost: $150–$400

Failed or stuck contactor

Pro

Silent while the thermostat calls, or a click with no start; burnt or pitted contacts, sometimes ants

Typical cost: $150–$300

Failed condenser fan motor

Pro

Compressor runs but the fan doesn't spin; unit gets very hot and shuts off

Typical cost: $400–$800

Compressor overload tripped

DIY or pro

Unit shuts off on hot days and restarts later; often a dirty coil or bad fan behind it

Typical cost: $100–$300 coil cleaning, or the underlying repair

Failed compressor

Pro

Fan runs, compressor won't start, breaker trips; usually on older systems

Typical cost: $1,500–$3,000

Cause	Signs	Who fixes it	Typical cost
Tripped breaker or blown disconnect fuse	Completely silent, breaker tripped	DIY or pro	$0 to reset; service call if it trips again
Failed capacitor	Humming, fan not spinning or starting slowly; a bulging or leaking capacitor	Pro	$150–$400
Failed or stuck contactor	Silent while the thermostat calls, or a click with no start; burnt or pitted contacts, sometimes ants	Pro	$150–$300
Failed condenser fan motor	Compressor runs but the fan doesn't spin; unit gets very hot and shuts off	Pro	$400–$800
Compressor overload tripped	Unit shuts off on hot days and restarts later; often a dirty coil or bad fan behind it	DIY or pro	$100–$300 coil cleaning, or the underlying repair
Failed compressor	Fan runs, compressor won't start, breaker trips; usually on older systems	Pro	$1,500–$3,000

Typical U.S. installed prices in 2026. Most repairs also carry a service call fee of $75–$200, often credited toward the repair. See AC repair costs in 2026 for the full list.

Step by step

1
Confirm it's being asked to run
Set the thermostat to Cool, well below room temperature, and wait 5–10 minutes for any restart delay. If the indoor fan runs, the thermostat is calling.
2
Check the breaker and disconnect
Reset a tripped breaker once. Check the disconnect box beside the unit is on. If it has fuses, a technician can test them.
3
Listen and look
Silent usually points to power or the contactor. Humming with a still fan usually points to the capacitor. Fan off but compressor running points to the fan motor. Tell the technician what you heard; it speeds up the diagnosis.
4
Turn it off if it hums
If the unit hums without starting, switch it off at the thermostat and the disconnect. A motor trying to start against a failed capacitor overheats quickly.
5
Clean around it
With power off, clear leaves, grass and debris, and rinse the fins from the outside. A clogged coil makes the compressor overheat and trip its overload.

Call a technician if

The unit hums but doesn't start
The fan doesn't spin while the compressor runs
The breaker trips again after one reset
You see burn marks on wires or the contactor

Red flags on a quote

A capacitor priced well above $400 installed. The part itself is inexpensive.
A compressor or full system recommended without testing the capacitor and contactor first
For a compressor on a system over about 10 years old, compare the quote with replacement using the repair vs replace calculator before agreeing

Facing an expensive repair on an older system? Run it through the repair vs replace calculator, or read the $5,000 rule.

Frequently asked questions

Why is my AC outdoor unit not running but the indoor fan is?

The most common causes are a tripped breaker, a failed capacitor or a failed contactor. A capacitor failure usually makes the unit hum without the fan spinning; a contactor problem usually leaves it silent. Both are common, relatively cheap repairs ($150–$400).

Why is my AC outdoor unit humming but not running?

Usually the capacitor that gives the fan motor and compressor their starting boost has failed. Turn the system off: a motor straining to start will overheat. A capacitor replacement typically costs $150–$400 installed.

Should I keep running the AC if the outdoor fan doesn't spin?

No. Without the fan, the compressor can't get rid of heat and will overheat, trip its overload and, over time, fail. Turn it off and have the fan motor or capacitor checked.