Contactor
An electrically operated switch that connects mains power to the compressor and outdoor fan when the control system calls for them. Its contacts carry the full load current, so when they pit or weld the outdoor unit can fail to start or fail to stop. Checking one means opening a live electrical compartment, which makes it technician work.
Also called relay
What does a contactor do?
A contactor is a heavy duty relay that connects mains power to the compressor and outdoor fan when the controls call for them. Its contacts carry the full running current of the outdoor unit.
Contactor: how it works
A small control voltage energises a coil, the coil pulls an armature, and the armature closes contacts carrying the full load current to the outdoor unit. Every closure draws a small arc, so contact surfaces erode over years of cycling. Eroded contacts add resistance and heat, and contacts that arc-weld together leave the unit running with no way for the controls to stop it.
Where you meet the term
- In an outdoor unit that will not start even though the thermostat is calling.
- In an outdoor unit that will not stop, which is the signature of welded contacts.
- In a service report, where burnt or pitted contact faces are described alongside the replacement.
How to check it
- Note whether the unit responds to the controls at all. A unit that never starts and a unit that never stops point at opposite contactor failures, and both are observations an owner can report accurately. Owner check
- Verify control voltage at the coil before condemning it. A contactor that is not pulling in may be a healthy contactor with no signal reaching its coil, which moves the search to the control side. Technician
Contactor: what it is not
- A contactor that does not pull in is not proof the contactor has failed. No control signal produces the same result.
- The clicking sound is the coil, not the contacts. Hearing a click does not confirm that power reached the compressor.
Contactor questions
Why does my outdoor unit keep running after the thermostat is satisfied?
Contacts that have arc-welded closed keep mains power connected regardless of what the controls ask. Isolating at the breaker stops it, and the contactor is replaced rather than cleaned.
What is the difference between a contactor and a relay?
They work the same way. Contactor describes the heavy duty version built for the load current of a compressor, and relay is the general term for the smaller switching devices elsewhere in the system.
Can insects cause a contactor fault?
Yes. Contactors are a known nesting site, and debris between the contact faces prevents proper closure, which is one of the reasons the compartment is inspected during service.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Mini-Split Compressor Not Turning OnTrane ductless outdoor operation, breaker stop points, airflow observations, and compressor service limits.