Sub-Zero Code 15 4 00: Defrost Relay is stuck open

Code 15 4 00 means: Defrost Relay is stuck open. No power to defrost heater. Verify, replace control board. It falls under relay fault (stuck open or closed) — Relay codes mean a switch on the control board that turns a component on and off — the compressor, defrost heater, lights, icemaker, or a water valve — has gotten stuck in one position instead of switching normally.

Rarely — a physically stuck or welded relay contact is a mechanical failure that won’t resolve on its own, though power-cycling occasionally clears a marginal one temporarily.

It depends which relay is affected. A stuck-on compressor relay just means the compressor runs more than it should; a stuck-on defrost heater relay can cause real cooling loss, so have it checked promptly.

Power-cycling at the breaker sometimes clears the fault temporarily, but if the relay contact is physically stuck, the code will return.

In most cases yes, since the relay is soldered onto the board rather than a separate replaceable part.

It depends on which component it's stuck controlling — a technician can tell you quickly during diagnosis whether it's safe to wait or worth scheduling sooner.