AC Outdoor Unit Humming but Not Running: Capacitor, Contactor, Fan Motor or Compressor?

By HVAC Cheat Sheet Team · 3 min read

A humming outdoor unit has power. The contactor has closed and voltage is getting to the compressor, the fan motor or both, but something isn't turning. That narrows the problem a lot. If it's only the compressor that won't start, AC compressor not starting goes deeper on that branch.

Safety: Don't let it sit there humming. A compressor that can't start draws locked-rotor amps and heats up fast until its internal overload opens. Kill power at the disconnect, then discharge the capacitor before you touch anything.

What's humming?

What you see and hear Most likely Check
Fan not turning, compressor not running, loud hum Weak or failed capacitor (often the common side of a dual), or a seized fan motor Capacitor, then spin the fan
Fan running, compressor humming but not starting HERM side of the capacitor, a hard-starting or locked compressor, or low voltage Capacitor, supply voltage, then compressor
Fan humming but not turning, compressor running FAN side of the capacitor or a seized fan motor Capacitor, then the fan bearings
Buzz or chatter at the contactor, nothing starts Low 24 V to the coil, or pitted contacts Coil voltage and contact voltage drop
Hums, then trips the breaker Locked or grounded compressor, a shorted wire Stop and ohm the compressor

Check in this order

  1. Capacitor. It's the most common cause and the cheapest fix. With power off and the capacitor discharged, measure each side with a meter. See how to test a run capacitor. The part finder tells you whether your reading is in tolerance and lists replacements.
  2. Fan motor. With power off, spin the blade by hand. It should turn freely and coast. If it's stiff or won't turn, the bearings are gone: see condenser fan not spinning and how to replace a condenser fan motor.
  3. Contactor. Check the coil gets 24 V on a call, then measure the voltage drop across each closed contact under load. Burned or pitted contacts can pass enough voltage to hum but not enough current to start. See how to test a contactor.
  4. Supply voltage. Measure line voltage at the contactor while the unit tries to start. A big sag points at the supply, a loose connection or a failing breaker.
  5. Compressor. If the capacitor and contactor are good and the compressor still hums without starting, ohm the windings and check for a ground. See how to test a compressor. Good windings but no start usually means a mechanically tight compressor; a hard start kit may get it running.

Common mistakes

  • Leaving it humming while you diagnose. Every attempt to start a locked compressor heats the windings.
  • Swapping the compressor before testing the capacitor. A failed capacitor makes a good compressor hum and trip.
  • Testing only one side of a dual capacitor. Measure both HERM to C and FAN to C.
  • Ignoring why the capacitor failed. High head pressure from a dirty condenser or a slow fan kills capacitors too.

FAQ

Why is my AC humming but the fan isn't spinning?

Usually a failed capacitor or a seized fan motor. With power off, spin the blade by hand: if it turns freely, test the capacitor; if it's stiff, the motor's bearings have failed.

Can a bad capacitor make the compressor hum?

Yes. Without its capacitor, a PSC compressor can't start. It hums, draws locked-rotor amps and trips its internal overload.

Is it safe to push the fan blade to get it started?

Don't. If spinning it by hand starts the fan, the capacitor or motor is bad and needs replacing. Reaching into a powered unit is how people get hurt.

The compressor hums and trips the breaker. Is it dead?

Not always. Test the capacitor and the windings first. A grounded or open winding means the compressor has failed; good windings and a good capacitor point to a locked or hard-starting compressor.

Sources

Parts:Run CapacitorContactorCompressorCondenser Fan Motor
Symptoms:AC Not CoolingOutdoor Unit Won't StartOutdoor Fan Not Running

Tools for this job

PARTS

Replacement Part Finder

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