Air-conditioner contactor switches high-voltage power to compressor and fan. Related symptoms include outdoor unit silent, buzzing, chattering or running while off. Those symptoms do not prove the component failed: confirm thermostat demand and external disconnect/breaker state. Set the thermostat to cool and a few degrees below the room temperature and check that its display shows a call for cooling.
What this covers
- central AC and heat pumps
What it does not cover
Differential
Likely causes of “Air-conditioner contactor”, and how to tell them apart
- 1
The thermostat is not calling for cooling
What separates it The outdoor unit is silent because the thermostat is set to off, to heat or to a temperature above the room, or its batteries are low, so no signal reaches the contactor.
How to confirm Set the thermostat to cool and a few degrees below the room temperature and check that its display shows a call for cooling.
- 2
The outdoor disconnect or the breaker is off or tripped
What separates it Nothing runs outside and the indoor blower may also be off, and the disconnect handle near the unit is pulled or the breaker has moved.
How to confirm Check the breaker and the outdoor disconnect position once, and do not open the unit.
- 3
A safety switch or a float switch has stopped the system
What separates it The system stopped after water appeared near the indoor unit and the condensate pan or float switch is full.
How to confirm Look at the condensate pan and drain for water and clear only what the maker allows.
- 4
The contactor itself is worn, pitted or chattering
What separates it A buzzing or chattering sound comes from the outdoor unit's electrical compartment, or the unit keeps running after the thermostat is turned off.
How to confirm This can only be confirmed by a qualified technician with a meter; do not open the electrical compartment.
Before you touch a setting
Record this first — changing settings destroys the evidence
- What the thermostat shows and whether the indoor blower runs when cooling is called for.
- The position of the breaker and the outdoor disconnect, and any sound the outdoor unit makes.
- The model and serial number on the outdoor unit's rating plate.
Try in this order
0 of 2 completedTest before you replace
ReversibleConfirm thermostat demand and external disconnect/breaker state.
Weigh the repair
ReversibleReplacement difficulty for the air-conditioner contactor is professional high-voltage work.
Stop boundary
When this stops being a self-service repair
- !Do not open the outdoor unit's electrical panel; a contactor switches the full line voltage and replacement is professional high-voltage work.
- !Turn the unit off at the breaker and call a technician if it runs when it should be off, or buzzes and chatters repeatedly.
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Can I replace a contactor myself?+
The part is inexpensive, but it switches high voltage and the wrong replacement or a missed step can injure you or damage the compressor. It is a job for a qualified technician.
Does a buzzing outdoor unit always mean a bad contactor?+
No. A weak capacitor, a jammed fan or a low-voltage supply can also cause buzzing. Confirming which needs measurement.
What does the contactor do?+
It is a switch that sends power to the compressor and fan when the thermostat calls for cooling.
What signs suggest a bad one?+
The unit does not start, buzzes, or runs continuously because the contacts are pitted or stuck.
Should I open the panel?+
Only a qualified technician should, since high voltage is present.