Outside AC unit not running but inside is: causes

Safety first. Do not open the outdoor unit or push the fan blade with a stick to start it. The capacitor inside stores a dangerous charge with the power off, and the fan can start without warning.

Check these first

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Confirm the thermostat is on Cool with the setpoint below room temperature, then wait a few minutes for the compressor start delay.
Check the AC breaker in the main panel. It is usually a double-pole breaker. Reset it once if it has tripped.
Check the condensate drain pan at the indoor unit. A full pan can trip a float switch that shuts off only the outdoor unit.
Stand by the outdoor unit and listen: silence, a click, a hum, or a fan running with no compressor sound all mean different things.
Look through the top grille. Note whether the fan is spinning, still, or blocked by a stick or debris you can see.
Clear leaves and debris from around the unit, and look for a chewed or cut thermostat wire where it enters the unit.
If the unit hums or buzzes without starting, turn the thermostat to Off.

Still not working after these? That is the point to bring in a licensed technician.

Request a call back Repair or replace?

What it usually means

The indoor blower and the outdoor unit are powered and controlled separately. The blower can run happily while the outdoor unit sits dead. What the outdoor unit is doing narrows the cause:

What you see or hear	Most likely cause
Completely silent	Tripped breaker, disconnect off, float switch, blown low-voltage fuse, damaged thermostat wire
Click, then nothing	Contactor pulling in with no power behind it, or a failed capacitor
Humming, fan not spinning	Failed capacitor or seized fan motor
Fan spinning, no cooling	Compressor not starting: capacitor, overheated compressor, or compressor failure

Likely causes, most common first

Failed capacitor. The capacitor gives the compressor and fan motor the boost they need to start and run. When it fails, one or both just hum. It is the most frequent outdoor-unit repair. See what a capacitor replacement involves.
Tripped breaker or pulled disconnect. One reset is fine. A second trip means a real fault.
Float switch or blown fuse. A clogged drain or a shorted thermostat wire removes the start signal.
Worn contactor. Burned or pitted contacts stop power from reaching the motors.
Condenser fan motor. A motor with worn bearings stalls or runs slowly, and the compressor then overheats and shuts off. See the condenser fan motor replacement guide.
Compressor. A compressor that has overheated will restart after it cools. One that is shorted or seized will not, and it often trips the breaker.

What a technician will do

The technician checks for line voltage at the disconnect and the low-voltage start signal at the contactor. With power off and the capacitor safely discharged, they measure it against its rated value. They check the fan motor for free spin and the compressor windings for shorts or opens, and they measure amp draw on startup.

A compressor is not condemned on sound alone. Many units diagnosed over the phone as a dead compressor turn out to need a capacitor or a contactor. Ask to see the meter readings.

If a capacitor or fan motor has failed, a careful technician also looks for the reason. A dirty condenser coil, a failing motor that pulled too much current, or voltage problems can all shorten the life of a new part. If the cause was a drain float switch or a damaged thermostat wire, the fix is at the drain line or the cable, and the outdoor unit itself is fine.

You cannot test any of these parts yourself. What you can do is report what the unit was doing, whether the breaker had tripped, and whether the problem came on suddenly or after days of hard starting.

When it points to replacement

Capacitors, contactors and fan motors are routine repairs on a system of any age. The picture changes if the compressor has failed. It is the most expensive part in the outdoor unit, and on an older system out of warranty the repair can approach the price of a new outdoor unit. The compressor replacement cost guide covers when that repair stops making sense.

Common questions

Why is my AC fan not spinning outside?

If the unit hums and the fan stays still, the capacitor or the fan motor has most likely failed. If it is silent, the unit has no power or no start signal. Turn the system off either way. A compressor running without the fan overheats quickly.

The condenser fan is spinning but the compressor is not turning on. What is wrong?

The fan and compressor share a dual capacitor in many units, and the compressor side can fail alone. An overheated compressor that has tripped its internal protector behaves the same way until it cools. A technician can tell these apart from a failed compressor with a meter.

Is it safe to reset the outdoor AC breaker?

One reset is reasonable. Switch the breaker fully off, then on. If it trips again, leave it off. Repeated resets push current through a shorted wire, motor or compressor and can cause more damage or a fire.

Should I leave the indoor fan running if the outside unit is not working?

Running the fan alone does no harm and helps air circulate, though it will not cool the house. Set the thermostat's cooling mode to Off so the outdoor unit is not repeatedly trying to start while you wait for service.

Next step

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Outside AC unit not running (but inside is)