What a Run Capacitor Does, and Why It Fails
The run capacitor helps the compressor or fan motor start and keep running. A failed one is a common outdoor-unit fault. The part stores energy, so it is not a homeowner swap.
The run capacitor on an outdoor unit is a small metal can, often dual-section: one side for the fan and one side for the compressor. It does not store cooling. It stores an electrical charge that shifts the motor's start winding. With a good capacitor the motor starts cleanly and draws the current it was designed for. With a weak one the motor strains, heats, and may not start at all.
Signs at the outdoor unit
- A hum from the condenser and a fan that does not turn.
- The compressor clicks or hums and the breaker trips.
- Cooling that is weak because the fan is turning slowly or the compressor is not staying on.
Heat and age are the usual reasons capacitors fail. A unit that sits in full afternoon sun works the part harder. A capacitor that is bulged or leaking oil has failed visibly. One that looks perfect can still be out of its microfarad range. Looks are not the test.
Leave the can alone
Even with the disconnect pulled, a capacitor can hold a charge that will arc. Shorting the terminals with a screwdriver can injure you and is a bad way to discharge it. The replacement has to match both the microfarad rating and the voltage. A dual capacitor has to match both sections. A compressor that has been starting against a bad capacitor may already be damaged. Putting a new capacitor on a shorted compressor just trips the breaker again.
If the outdoor unit hums or will not start, shut it off at the thermostat so it is not trying, and book AC repair. The capacitor, the contactor, and the motors get tested together. A new air conditioner is not the conclusion of a bad capacitor.
Common questions
Is a run capacitor the same as a start capacitor?
A run capacitor stays in the circuit while the motor runs. A start capacitor is only in the circuit for a moment. Many residential condensers use a run capacitor, sometimes with a separate start device.
Will a new capacitor fix a dead compressor?
Only if the capacitor was the failed part. A shorted compressor will not start on a new capacitor.
Can I buy the same-looking capacitor online?
The microfarad and voltage ratings have to match, and the old one can still be charged. Have a technician test and replace it.
