AC Humming or Buzzing but Not Starting? It May Be the Capacitor

If your outdoor AC unit is making a humming or buzzing sound but the fan isn't spinning, or the house just isn't getting cool, one of the most common causes is a failed capacitor.

What Does an AC Capacitor Do?

The capacitor gives your compressor and outdoor fan motor the jolt of energy they need to start, then helps keep them running. Extreme heat wears capacitors out, which is why they fail so often during long San Antonio summers.

Signs of a Bad Capacitor

• Humming or buzzing from the outdoor unit, but the fan doesn't turn
• Indoor fan runs, but the air coming out isn't cold (see AC blowing warm air)
• The AC takes a long time to start, or shuts off on its own
• Clicking sounds when the system tries to start
• A swollen or leaking capacitor (visible only with the panel off, leave that to a tech)

Why You Shouldn't DIY It

Capacitors hold an electrical charge even after the power is off and can cause serious injury. Online videos make it look simple, but a trained technician will safely discharge it, test it, and also check the contactor and fan motor, the parts that often fail alongside it.

Turn the System Off

If the unit is humming but not running, turn the thermostat off. Letting the compressor keep trying to start can overheat it and turn a small repair into a big one.

Is It Something Bigger?

Sometimes humming points to a failing fan motor or compressor. Our $129 diagnostic (waived if you go ahead with the repair) tells you exactly what's wrong. If your system is older and the compressor is failing, compare the repair with a free replacement estimate. For more warning signs, see signs you need AC repair.