Outdoor unit won't start: guided troubleshooting
Thermostat is calling and the indoor blower runs, but the condenser or heat pump outside doesn't. Answer each question from what you see on site.
Step 1
What is the outdoor unit doing?
See every step in this flow
- What is the outdoor unit doing?
- Nothing at all, silent: Is there line voltage at the top (line side) of the contactor?
- Humming or buzzing: Which part is running?
- Clicking on and off: Contactor chattering
- Tripping the breaker: Tripping the breaker
- Is there line voltage at the top (line side) of the contactor? Measure L1 to L2 with the disconnect in.
- Yes: Is there 24 V across the contactor coil?
- No: No line voltage at the unit
- Is there 24 V across the contactor coil?
- Yes: Is the contactor pulled in, with line voltage on the load side?
- No: No call reaching the outdoor unit
- Is the contactor pulled in, with line voltage on the load side?
- No, it isn't pulling in or isn't passing voltage: Contactor
- Yes, voltage on the load side and still nothing runs: Power is getting through, nothing runs
- Which part is running?
- Neither fan nor compressor: Start with the run capacitor
- Fan runs, compressor hums or is silent: Compressor isn't starting
- Compressor runs, fan doesn't: Condenser fan isn't running
Where each path ends
No line voltage at the unit
Check the breaker, the disconnect and its fuses, and the whip. A breaker that won't reset needs the short found before it's reset again.
No call reaching the outdoor unit
Follow the 24 V back from the contactor coil: pressure switches and time-delay in the unit, the thermostat wire, a float switch in the Y circuit, then the indoor board.
Contactor
With 24 V on the coil, a contactor that doesn't pull in has an open coil. One that pulls in but drops voltage across the contacts is pitted. Replace it.
Power is getting through, nothing runs
Check wiring from the contactor to the capacitor, compressor and fan. A compressor that is hot and silent may be out on its internal overload and needs time to reset.
Contactor chattering
Low or unstable 24 V at the coil: a weak transformer, a loose or damaged thermostat wire, or a safety switch opening and closing.
Tripping the breaker
Don't keep resetting it. Check for a grounded compressor or fan motor, a shorted capacitor or wire, and breaker size against the nameplate.
Start with the run capacitor
Both motors humming and neither starting usually means a failed dual run capacitor. Test it with the power off and the capacitor discharged.
Compressor isn't starting
Test the compressor side of the capacitor, then voltage at the compressor terminals, then the windings.
Condenser fan isn't running
Shut the unit down before head pressure climbs. Test the fan side of the capacitor, then the motor.