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How do you replace a start/run capacitor in an AC?

Updated August 2026

30–45 minutes · Advanced DIY

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The parts bill is small, $10 to $30 and 30 to 45 minutes, but the capacitor holds a charge after the power is off, so discharging it comes first. A failed dual run capacitor is the leading reason a condenser hums without starting, or the fan sits still while the compressor labors. Two things decide whether the swap goes cleanly: a photo of the wiring before you touch it, and a replacement whose microfarad values match the old label exactly.

What you need

Before you start: A run capacitor stores a charge that can still injure you days after the power is off, and the cabinet it sits in is fed by 240 volts. Pull the disconnect, kill the breaker, then discharge across each pair of terminals with an insulated screwdriver before a finger, a tool, or a wire goes near them. Keep one hand in your pocket, stand on dry ground, and never open the cabinet in the rain. A capacitor with a domed top or oily residue has already failed and can vent, so glove up and take it to household hazardous waste. If this reads as beyond you, it is, and $150 to $400 for a tech is a fair price.

Steps

  1. Set the thermostat to off, pull the outdoor disconnect block, and switch off the indoor breaker for the condenser. Two shutoffs, not one.
  2. Wait 5 minutes, then remove the screws holding the electrical access panel on the side of the cabinet.
  3. Discharge the capacitor before anything else. With an insulated-handle screwdriver, bridge the shaft across the C and HERM terminals, then across C and FAN. Expect a small snap. Do this every time, even when you are certain the power is off.
  4. Photograph the wiring from two angles. A dual run capacitor has three terminal groups: HERM for the compressor, FAN for the condenser fan motor, and C for common. Wire colors vary by brand, so the photo is your only reliable record.
  5. Read the old label and write down both microfarad values and the voltage, for example 45/5 uF at 370 VAC. The new can must match those uF numbers within the printed tolerance, usually 6%, and carry the same or a higher voltage rating.
  6. Move the wires one terminal at a time with needle-nose pliers, gripping the connector and never the wire.
  7. Loosen the mounting strap, seat the new capacitor, and tighten the strap so the can cannot vibrate against the cabinet.
  8. Close the panel, restore power at the breaker and the disconnect, and allow 5 minutes for the compressor time delay before calling for cooling.
  9. Watch the start. The fan should come up to speed on its own and the compressor should settle into a steady hum within a second or two. If it hums and trips out again, stop: the fault is upstream in the contactor, the wiring, or the compressor.

Or hire it out

A tech charges $150 to $400 to swap a capacitor, and a peak-season or after-hours call can reach $500 for a part that costs $10 to $30 online. That gap is why some homeowners keep a matching spare on the shelf, then hand it over anyway when the diagnosis turns out to be the contactor.

Done. Now the part everyone skips: doing it again on schedule. Log it in OnOtto and the next reminder for AC capacitor checks sets itself.

Log this task, get the next reminder

Frequently asked questions

How do I know the capacitor is the actual problem?

Look, then measure. A domed or split top, or oily residue around the base, is a failed capacitor with no further testing needed. Otherwise discharge it, disconnect the wires, and read it on a meter with a capacitance range: more than 6% below the rated uF means it is worn out. The classic symptom is a condenser that hums a few seconds then clicks off. What an HVAC capacitor is explains the two ratings on the label.

Can I install a higher microfarad rating while I am at it?

No. The microfarad value is matched to the motor windings, and a larger one pushes current the motor was never designed for, running it hot and shortening its life. Match both numbers on a dual can, for instance 45/5, within the printed tolerance. Voltage is the one rating you may go up on: a 440 VAC capacitor substitutes fine for a 370 VAC one, never the reverse. Can size does not matter as long as the strap holds it firmly.

Why does my capacitor keep failing?

Heat, usually. Capacitors carry a temperature rating, and a unit baking in full afternoon sun in Phoenix may see 5 years where a shaded one in Ohio sees 10. Dirty coils make it worse by raising head pressure and forcing longer run times, which is one more argument for the yearly coil rinse. Repeat failures inside two years point at a failing contactor or a compressor drawing high amps. Why an AC capacitor keeps failing goes deeper.

What happens if I keep running the AC on a weak capacitor?

You trade a $10 to $30 part for a much larger one. A weak capacitor makes the compressor draw locked-rotor current on every start attempt, which cooks the windings and can take out a compressor worth $1,500 to $2,800 installed. The fan motor suffers the same way at $300 to $700. The cost of ignoring a failing capacitor lays out the ladder, and replacement intervals cover proactive swaps.

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