What the contactor does
A low-voltage signal energizes the contactor coil. The contacts close and deliver high voltage to the outdoor equipment.
When the thermostat call ends, the coil de-energizes and the contacts should open.
Common symptoms
The outdoor unit may not start, may chatter, may operate intermittently, or may continue running after the thermostat is satisfied.
Pitted or burned contacts can create voltage drop and heat even when the system still runs.
Why contactors fail
Every opening and closing cycle creates a small electrical arc. Over time, the contact surfaces wear.
Loose connections, insects, dirt, low coil voltage, high motor current, and repeated short cycling can accelerate failure.
How it is diagnosed
Technicians inspect the contacts, verify coil voltage, check voltage drop across closed contacts, and evaluate current draw.
The cause of severe burning should be investigated before a new contactor is installed.
Repair considerations
A contactor is normally replaced rather than repaired. The replacement must match the coil voltage and electrical rating.
Associated wiring, capacitors, motors, and the compressor should be checked for abnormal load.
Frequently asked questions
Can a contactor buzz?
Yes. Buzzing or chattering can indicate voltage, coil, contamination, or mechanical problems.
Can contacts weld shut?
Yes, and that can keep equipment running when it should be off.
Can insects cause failure?
Yes. Debris between contacts is a common outdoor-unit problem.
Is a burned contactor the whole diagnosis?
Not always. Excess current or low voltage may have contributed.
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