What Is a Start Relay?

A start relay is a small electrical component that gives the compressor motor the extra jolt of current it needs to start spinning, then switches out of the circuit once the motor is running. It's mounted directly on the compressor, usually alongside the overload protector, and is one of the least expensive parts in the sealed system.

A failed start relay is a common reason a compressor "hums but won't start" — you'll hear an attempt every few minutes, sometimes with an audible click, but the compressor never actually kicks in. It's worth ruling out before assuming the compressor itself has failed.

Frequently Asked Questions

How can I tell if my compressor's start relay is bad?

The classic sign is a compressor that hums for a few seconds, then clicks off, repeating every few minutes without ever fully starting. A technician can confirm it by testing the relay directly, since a seized compressor can produce a similar pattern.

Is a start relay an expensive part to replace?

It's one of the least expensive components in the sealed system, and swapping a genuinely bad relay is a straightforward repair — which is exactly why it's worth ruling out before assuming a far more expensive compressor replacement is needed.