E4 error displayed; freezer shows excessive frost or, in some models, no cooling with defrost heater constantly on.

Safety: Disconnect mains power. The defrost heater operates at mains voltage and can be hazardous.

• Defrost and access: Remove food, manually defrost the freezer, then remove the internal rear panel to expose the evaporator and defrost components.
• Locate heater and safety devices: The defrost heater is usually a metal or glass-encased element at the base or woven through the evaporator. A thermal fuse or defrost thermostat is often clipped to the evaporator tubing.
• Test the heater: Disconnect heater connectors and measure resistance with a multimeter. A typical reading is between 50–300 Ω depending on model. Infinite resistance indicates a failed heater.
• Check thermal fuse/thermostat: Test for continuity at room temperature. An open circuit suggests the safety device has blown and must be replaced with the correct Zanussi-rated part.
• Inspect wiring: Look for burnt connectors, melted insulation, or broken wires around the heater and thermostat.
• Replace faulty parts: Install a new defrost heater and/or thermal fuse/defrost thermostat as needed. Ensure good thermal contact with the evaporator where required.
• Control board check: If all components test good, the main control PCB may not be energizing the heater correctly. Board replacement may be necessary.

After repair, reassemble, restore power, and monitor for normal frost patterns and absence of the E4 code.