WiremanWireman · A/C outdoor unit won’t run: troubleshooting for electricians · 2023 NEC ·
A/C outdoor unit won’t run
Split it at the contactor: is the 24 V call arriving, and is 240 V getting through?
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How it works
When the thermostat calls for cooling, it connects R to Y. That 24 V signal travels from the furnace or air handler out to the condenser and energizes the contactor coil, which closes and sends 240 V to the compressor and fan. The indoor blower runs separately on G. So the outdoor unit needs two things at once: line voltage from its own breaker and disconnect, and the 24 V call from the indoor unit’s transformer.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- With the thermostat set to cool, is the indoor blower running?
- Yes, air is blowing insideTest: Measure 24 V at the contactor coilExpect: About 24 V AC (often 24 to 28 V).
- 0 V at the coilTest: Check Y at the indoor boardExpect: About 24 V while cooling is called for.
- 24 V at Y indoorsOpen Y circuit between the indoor unit and the contactor
- 0 V at Y indoorsThe thermostat isn’t sending a cooling call
- 24 V, contactor not pulled inFailed contactor coil or stuck contactor
- 24 V, contactor pulled inTest: Measure line and load side of the contactorExpect: About 240 V (or 208 V) on both sides.
- No voltage on the line sideTest: Check the disconnectExpect: About 240 V coming in and going out.
- Voltage in, none outBlown fuses or a bad disconnect
- No voltage coming inNo power from the panel
- Line side good, load side low or 0Burned or pitted contactor contacts
- Both sides goodWith 240 V on the load side, what do the compressor and fan do?
- Fan runs, compressor doesn’tThe compressor isn’t starting
- Compressor runs, fan doesn’tThe condenser fan isn’t running
- Both hum, neither startsLikely a failed dual run capacitor
- Silent, nothing tries to startOpen wiring after the contactor
- No voltage on the line sideTest: Check the disconnectExpect: About 240 V coming in and going out.
- 0 V at the coilTest: Check Y at the indoor boardExpect: About 24 V while cooling is called for.
- No, nothing runs inside or outNo 24 V control power
- Thermostat is blankNo 24 V control power
- Yes, air is blowing insideTest: Measure 24 V at the contactor coilExpect: About 24 V AC (often 24 to 28 V).