Display shows E4 and the unit may not defrost, leading to heavy frost on the evaporator and poor cooling.

Safety first: Disconnect power. The defrost circuit operates at line voltage and can cause shock if energized.

Access the evaporator and defrost components:

• Empty the freezer and remove the rear interior panel.
• Locate the defrost heater (usually a metal or glass-encased element at the bottom or woven through the evaporator) and the defrost thermostat/thermal fuse clipped to the coil.

Test the heater:

• Unplug the heater leads and check continuity with a multimeter. An open circuit means the heater is bad and must be replaced.

Test the defrost thermostat:

• If the thermostat is cold (covered in frost), it should show continuity. If it is open when cold, it is defective.

Inspect wiring and board:

• Check for burned connectors or melted insulation in the defrost wiring.
• At the main control board, inspect the defrost relay area for burn marks.

Repair: Replace any failed components (heater, thermostat, or thermal fuse) with Summit-compatible parts. If all components test good, the main control board defrost relay may be faulty and should be replaced.