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What are the symptoms of a bad AC capacitor?
The classic sign is an outdoor unit that hums while its fan sits dead still — and then starts spinning if you nudge a blade with a stick. Capacitors store the electrical kick that gets the compressor and fan motors turning. When heat and age make them fail, the motors can't start: they stall, draw enormous current, and trip the breaker. A swollen, domed top on the capacitor is a dead giveaway.
What causes it
Capacitors store and release the electrical jolt that starts and runs the compressor and the fan motors. Heat and age make them weaken, bulge, and fail — it's one of the single most common AC breakdowns, especially in a heat wave.
What it does to the cycle
Without the capacitor's boost, a motor can't get going or keeps stalling. The outdoor fan may sit dead still while the unit just hums, or the compressor strains to start, pulls huge current, and trips the breaker.
What you'd notice
A humming outdoor unit with a fan that won't spin (sometimes it'll start if you nudge a blade — a classic sign), a clicking or buzzing relay, no cooling on the hottest days, or a visibly swollen, domed top on the capacitor.
What to do about it
- Look at the top of the capacitor (a silver can in the outdoor unit's electrical panel). Domed or bulging means it's finished.
- Note the classic test: the unit hums, the fan won't spin, but a nudge with a stick starts it turning.
- Do not touch it. A capacitor holds a lethal charge for a long time even with the power off and the disconnect pulled.
- Have a tech discharge it safely and fit one matching the exact µF and voltage rating printed on the old can.
- It's a cheap part and a fast job — usually the best-value repair on the whole unit.
The bottom line
It's a cheap part, but it holds a dangerous charge even with the power off. A pro discharges it safely and installs one with the exact microfarad (µF) and voltage rating.