Feelin Good HVAC

Home / Guides

How to Tell If Your AC Capacitor Is Bad (and Replace It Safely)

The capacitor is one of the most common reasons an air conditioner quits on the hottest day of the year. It's a small cylinder inside your outdoor unit that gives the motors the jolt they need to start and keep running. The good news: it's a cheap part and a straightforward swap — if you respect the safety steps below.

Symptoms of a bad capacitor

  • The outdoor unit hums but the fan doesn't spin (sometimes you can nudge it with a stick and it takes off — that's a classic sign)
  • The AC blows warm air or the compressor won't kick on
  • The system tries to start, then shuts off after a few seconds
  • You open the panel and the capacitor looks swollen, bulged, or is leaking an oily fluid

Any one of these is worth a look. Capacitors wear out from heat and age — five to ten years is a typical life.

How to replace it

  1. Kill the power — twice. Turn off the breaker for the AC at your electrical panel, then pull the disconnect block (or flip the switch) at the outdoor unit. Both.
  2. Confirm it's dead. Check with a non-contact voltage tester or multimeter that there's no power at the unit before you touch anything. Trust the meter, not the switch.
  3. Open the electrical panel on the outdoor unit (usually a corner panel held by a few screws).
  4. Take a photo of the wiring before you touch anything. You'll thank yourself later.
  5. Discharge the capacitor. Even with the power off, a capacitor can hold a charge. Touch an insulated-handle screwdriver across the terminals to discharge it. You'll sometimes see a small spark — that's normal.
  6. Read the ratings printed on the side: something like "45/5 µF, 370V." Buy an exact match on the microfarad (µF) rating. The voltage rating must match or be higher — never lower.
  7. Move one wire at a time from the old capacitor to the new one, matching the terminals (usually marked HERM, FAN, and C). Your photo is the backup.
  8. Secure the new capacitor in the same strap or bracket, close the panel, restore power at the disconnect first, then the breaker.
  9. Test it. Set the thermostat to cool and listen for the outdoor unit starting up cleanly.

Call a pro when…

  • The wiring or contactor looks burnt or damaged
  • The new capacitor doesn't fix the problem (the motor itself may be shot)
  • You're not comfortable working around electrical components — that's a completely fine reason

Capacitors cost a fraction of a service call, and this is one of the most satisfying DIY wins in homeownership. For the video version, find me at HVAC DIY Help on TikTok and YouTube (@hvac_diy_help).