Display shows E4 and refrigerator experiences heavy ice build-up or poor cooling

Safety: Disconnect mains power before accessing the freezer evaporator or control board. Risk of electric shock and sharp metal edges.

Steps to diagnose:

• Check for ice build-up: Remove freezer drawers and rear panel. If the evaporator is a solid block of ice, the defrost system is likely not working.
• Inspect defrost heater: The heater is usually a metal or glass tube attached to the bottom or around the evaporator. Look for burn marks or breaks.
• Test heater continuity: Disconnect heater leads and measure resistance. An open circuit indicates a failed heater that must be replaced.
• Check defrost thermostat / thermal fuse: Locate the small round thermostat or inline fuse clipped to the evaporator. Test for continuity when the sensor is cold (should be closed). Replace if open when cold.
• Inspect wiring: Follow the harness from heater and thermostat to the main PCB. Look for burnt connectors or broken wires.
• Control board: If heater and thermostat test good, the defrost relay or control logic on the PCB may be faulty. This typically requires PCB replacement.

Warning: Defrost heater circuits operate at mains voltage. If you are not experienced with electrical testing, contact a qualified Zanussi technician.