Every summer, thousands of air conditioners die on the hottest week of the year — and in a huge share of those service calls, the culprit is a metal can the size of a soda can that costs less than dinner: the run capacitor.
What it does (in one sentence)
The capacitor gives your compressor and fan motors the electrical kick they need to start and keep spinning — when it weakens, motors hum, struggle, overheat, and eventually refuse to start at all.
Why it's almost always the capacitor
Capacitors live outside in the heat, absorb every voltage spike, and chemically age even when everything else is healthy. Techs will tell you it's the single most-replaced part in residential AC — which is exactly why every service truck carries a stack of them.
The warning signs
- The outdoor unit hums but the fan doesn't spin (give it a gentle push with a stick — if it takes off, that's a textbook dying capacitor)
- AC takes several tries to start, or trips the breaker on startup
- Warm air inside while the outdoor fan runs
- A capacitor with a bulged or domed top — that one's already gone
The 60-second test (for meter owners)
Kill power, discharge the cap, and measure microfarads against the rating printed on the side. The industry rule: more than 6% below rated MFD = replace it. Our free capacitor test tool does the math and gives you a pass/fail verdict instantly.
What it should cost
The part itself typically runs $15–$40. A contractor replacement commonly lands between $200 and $400 with the service call — fair for licensed diagnosis and warranty, but worth knowing when you're quoted more. DIY-inclined and comfortable with electrical safety? OEM and universal capacitors ship same-day, and matching the exact spec off your old cap takes one search in our Part Finder.
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