Replace an AC Run Capacitor

Goal

Replace a failed (or visibly bulged) run capacitor on a residential split-system air conditioner or heat pump. Capacitor failure is the single most common cause of a "won't start" or "humming compressor" service call - accounts for 30 - 40 % of summer no-cool tickets in a typical shop. A correct replacement takes 15 minutes; misdiagnosing one as a bad compressor costs a customer unnecessarily.

Prerequisites

• Replacement capacitor matched to the original by µF (microfarad) value AND voltage. Match all three values on a dual-run cap (HERM / FAN / COM).
• Insulated screwdriver
• Needle-nose pliers
• Capacitor discharge tool (or insulated screwdriver with a wooden handle)
• Multimeter with capacitance setting (recommended) or known-good cap tester
• Camera / phone (photograph the wiring before disconnecting - this is non-negotiable)

15 - 20 minutes per cap if the failure is straightforward.

Steps

1. Kill power at the disconnect AND the breaker. Verify dead with a voltage tester at the contactor before opening the panel.
2. Open the outdoor unit's service panel. The capacitor is the cylindrical (round or oval) component, typically near the contactor.
3. Photograph the existing wiring before touching anything. Get every label visible - HERM (compressor), FAN, C or COM (common).
4. Discharge the capacitor. Even with power off, a capacitor stores energy. Short the terminals to each other with the insulated discharge tool. A weak pop is normal.
5. Note the markings on the old cap: µF value for each terminal, voltage rating, polarity if applicable.
6. Disconnect the wires one at a time. Label each with tape if your photo isn't crystal-clear.
7. Remove the mounting strap or bracket and lift out the old capacitor.
8. Install the new cap. Match the orientation of the original (some have polarity; most run caps don't, but if it has a mark, follow it).
9. Connect wires per the photo / labels. HERM goes to compressor, FAN to fan motor, C to common.
10. Mount + secure with the bracket. A loose cap will fail again from vibration.
11. Close the service panel. Restore breaker, then disconnect.
12. Verify operation. Set thermostat to call for cool. Listen for the compressor to start cleanly without a labored hum.

Verification

• Compressor + outdoor fan start within 1 second of the contactor pulling in
• Amp draw on compressor reads within nameplate FLA
• No hum or whine after 30 seconds of runtime
• Discharge air temperature drops 18 - 22 °F below return air after 10 minutes

Troubleshooting

• System still won't start after replacement: verify the new cap matches the old in µF AND voltage. A 45+5 dual cap in a system that needs 50+5 will hum but not start.
• Capacitor fails again within weeks: ambient is too hot, the cap is undersized for the system, or the compressor is drawing high amps (worn bearings). Measure compressor amps under load before declaring the second cap "defective."
• Customer says it worked for an hour then died: contactor stuck closed, frying the cap during the off-cycle. Replace the contactor too.

References

• Manufacturer service literature for the specific equipment
• Capacitor manufacturer datasheets (Genteq, Mars, AmRad)
• Manuall internal: Annual HVAC System Maintenance