Aircon Contactor and Relay

A contactor is an electrical switch inside the outdoor unit that turns on power to the compressor and fan motor. When the indoor unit sends a start signal, the contactor closes to complete the power circuit and let current flow. Without a working contactor, the compressor never receives power — even if every other component is healthy.

A relay is a smaller switch that handles lower-power control signals between components. Together, the contactor and relay form the switching path that connects your indoor commands to outdoor action. When either one fails, the outdoor unit stays silent or responds inconsistently, because the power path is broken at the switch.

Contactors open and close thousands of times over their lifespan, and the contact surfaces gradually wear down from repeated arcing. When the contacts become pitted or corroded, the switch fails to close fully — the indoor unit runs normally, but the outdoor unit stays silent or clicks without starting. Some days the contactor closes successfully and others it does not, creating an intermittent pattern that confuses homeowners.

A weak capacitor produces symptoms identical to a contactor fault, because both prevent the compressor from starting. The clicking sound from the outdoor unit is the contactor receiving the start signal and attempting to close — hearing a click does not confirm the contactor is the problem. Only testing separates a worn switch from a weak capacitor or a control board fault that never sends the signal.

• –Indoor fan runs but outdoor unit is silent
• –Clicking sound from outdoor unit but no cooling
• –Outdoor unit works some days but not others

Diagnosis starts with the capacitor, because a weak capacitor is more common and mimics contactor failure closely. If the capacitor tests healthy, the technician checks whether the control board is sending the start signal to the contactor. When the signal arrives, the technician observes whether the contactor closes and completes the power circuit.

Visible burn marks or pitting on the contact surfaces confirm mechanical wear. If the contactor receives the signal but fails to close, the switch itself has failed and needs replacement. When no signal arrives at all, the fault sits upstream in the control board rather than the contactor.

Replace the contactor only after testing confirms the switch itself is broken and the capacitor has been ruled out. Contactor replacement is a low-cost repair. The real value lies in testing correctly to avoid swapping it when the capacitor or control board is the actual problem.

You can wait if the unit starts most of the time but occasionally fails on the first attempt — the contactor may still be functional enough for now, though an intermittent contactor tends to fail completely soon.

Do not wait if the outdoor unit fails to start on most attempts. Repeated failed starts put mechanical stress on the compressor, and early diagnosis avoids being stuck without cooling on a hot day.

Contactor replacement is a quick, straightforward repair once the fault is confirmed through testing. The part is common and usually available without delay.

Before approving any replacement, ask what specific test result confirmed the contactor as the fault. Proper diagnosis at this stage prevents paying for a contactor when the capacitor or control board is the real cause.