Plain-language explanation: This page is educational and does not replace testing the actual equipment.
What the contactor does
The thermostat and control circuit energize the contactor coil. The contacts then close and supply power to the outdoor equipment.
Common failure modes
Contacts can become pitted, burned, welded closed, or fail to close. The coil itself can also fail.
Common symptoms
The outdoor unit may not start, may chatter, may run when it should be off, or may operate intermittently.
Why it should be addressed
Damaged contacts can create voltage drop, heat, and unstable power delivery to expensive components.
Frequently asked questions
Can a contactor make buzzing sounds?
Yes. Buzzing or chattering can indicate coil, voltage, contamination, or mechanical problems.
Can a contactor stick on?
Yes. Welded contacts can keep equipment running even after the thermostat stops calling.
Is contactor replacement usually straightforward?
Often, yes, but the technician should also check the cause and inspect related electrical components.