Sub-Zero Error Code 15 4 01: Defrost Relay Stuck Closed

Sub-Zero code 15 4 01 on a Built-In (BI) unit means the defrost relay is stuck closed, so the defrost heater is receiving power continuously instead of only during scheduled defrost cycles. Constant heating fights the cooling system, raises compartment temperatures, and wastes energy. Sub-Zero's guidance is to verify the fault and, if confirmed, replace the control board, since the relay is integrated into it.

Because the heater is energized when it should not be, this fault can cause warming and unusual heat near the coil. It is a technician-level repair and should be addressed promptly.

The meaning of 15 4 01

The control board relay that powers the defrost heater is stuck in the closed position, meaning the heater stays energized at all times rather than only during brief defrost cycles. This continuous heat load opposes cooling, can drive up compartment temperatures, and increases energy use. The recommended action is to verify the stuck-closed condition and replace the control board if confirmed, because the relay cannot be serviced separately.

Most-likely causes

• Defrost relay welded or stuck in the closed position on the control board
• Control board damage from a power surge, heat, or age
• Failed relay driver circuit holding the relay energized
• Corrosion or contamination bridging the relay contacts
• Board fault causing continuous output to the heater (must be verified)

Safe owner checks

• Power cycle the unit at the breaker for about 5 minutes, then restore power and see if normal cooling resumes.
• Note any unusual warmth, condensation, or a warm spot near the back wall and report it to the technician.
• Check whether compartment temperatures are rising and move perishables if needed.
• Do not attempt to disconnect the heater yourself; arrange professional service.

When professional service is needed

Confirming a stuck-closed relay requires verifying that the defrost heater is energized outside of normal defrost timing, which involves live electrical measurement at the control board. A continuously powered heater is a heat-load concern that a qualified technician should resolve. The fix is typically a control-board replacement after verification.