What they do
The capacitor gives the compressor and fan motor the electrical kick they need to start, then helps them run efficiently. The contactor is a relay that switches high voltage to the outdoor unit when the thermostat calls for cooling.
Signs of failure
- Outdoor unit hums but the fan doesn't spin.
- Clicking with no start, or the unit runs when the thermostat is off (a stuck contactor).
- A capacitor with a bulged or leaking top.
- Timing: capacitors often fail in the hottest weeks of the year.
Why this is not a DIY job
Capacitors can store a high-voltage charge after the power is switched off, and the outdoor unit carries 240 volts. Leave testing and replacement to a qualified technician.
Cost
A capacitor replacement typically runs about $120–$400 and a contactor about $150–$450, depending on market and whether a service call is included. See the AC repair cost guide.
Frequently asked questions
Is a bad capacitor an emergency?
Not dangerous to leave alone with cooling switched off, but running a unit with a failed capacitor can overheat the compressor. Switch cooling off until it is repaired.
