Display shows E4 and the refrigerator may stop cooling periodically; often associated with heavy ice buildup on the evaporator.

Safety warning: Always disconnect the appliance from the mains before working on the defrost system. The heater operates at mains voltage and can cause electric shock.

Diagnostic procedure:

• 1. Inspect for ice: Remove freezer drawers and check the rear panel. If it is heavily frosted, the defrost system is likely not working.
• 2. Access evaporator and heater: Remove the rear freezer panel. The defrost heater is usually a metal or glass-encased element at the bottom or woven through the evaporator.
• 3. Test the defrost heater: Disconnect the heater leads. Use a multimeter to measure resistance. A typical heater will show a finite resistance (e.g., 50–300 Ω). If the reading is infinite (open circuit), the heater is faulty and must be replaced.
• 4. Check defrost thermostat/thermal fuse: Locate the thermostat clipped to the evaporator tubing. At freezing temperatures, it should show continuity. If it is open when cold, it is defective.
• 5. Inspect wiring and connectors: Look for burnt connectors, melted insulation, or broken wires between the heater, thermostat, and main control board.
• 6. Board function: If heater and thermostat test good and wiring is intact, the main control board may not be energizing the heater. This requires board replacement, best done by a technician.

Temporary measure: You can manually defrost by unplugging the unit and leaving doors open for 24 hours, but this does not fix the underlying fault.