Humming, fan not spinning: probably the capacitor
A humming outdoor unit with a stalled fan is the classic failed run capacitor. It is one of the cheapest HVAC repairs there is. Leaving the compressor trying to start against it can turn a $250 fix into a compressor.
What is probably happening
- The run capacitor, a small metal cylinder that gives the fan and compressor motors their starting kick, has failed. The motor hums but cannot start.
- Less often, the fan motor itself has burned out, or a fan blade is broken and out of balance.
- The part itself rarely costs more than $45. You are paying for the visit and the diagnosis.
Do this in the next five minutes
- Thermostat to OFF. Do not let it keep humming.
- Look through the top grille with the power off. A bulged or leaking can on top of the capacitor is a giveaway, but do not touch it.
- Check the filter and registers indoors so the visit is not wasted on airflow.
- Call for a same-day visit. Say: outdoor unit hums, fan does not spin. Most trucks carry the part.
Do not
- Do not push the fan with a stick to get it going. That is a common trick online and it puts your hand next to a 240 volt fan.
- Do not replace the capacitor yourself. It stores a charge even with the power off.
- Do not accept a whole-system replacement for a capacitor. Ask for the part and a price.
Who to call
A licensed HVAC technician
This is a quick swap for a technician who carries the part. It does not need a sales visit or a comfort advisor.
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What it should cost
$200 to $400
AC capacitor replacement installed, national range, averaging $250. The part rarely exceeds $45. A separate cost guide puts the full range at $100 to $250
Source: Angi, AC capacitor replacement cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Did you test the capacitor with a meter, and what did it read against its rating?
- Is the fan motor drawing normal amps now, or did the bad capacitor damage it?
- What is the total including the service fee?
- Is the new capacitor rated for the outdoor temperatures here?
End the call if
- A capacitor bill over $400 with no other work. The national range tops out there.
- A hard-start kit or compressor added on with no reading to justify it.