F4 code; heavy ice build-up and poor airflow; defrost cycle not operating correctly.

Safety: Unplug the freezer. The defrost circuit is at mains voltage.

• Defrost and access: Manually defrost and remove the rear internal panel to expose the evaporator.
• Locate thermostat/fuse: The defrost thermostat or thermal fuse is clipped to the evaporator tubing.
• Test continuity: With a multimeter, check for continuity across the device at room temperature (for fuse) or at cold temperature (for thermostat, place in ice water if needed). An open reading indicates failure.
• Inspect for overheating: Look for signs of melted insulation or discoloration around the heater and thermostat.
• Replace faulty device: Fit a new Zanussi-specified defrost thermostat/thermal fuse and clip it securely to the evaporator.
• Check heater and control: Also test the defrost heater and verify the control board initiates defrost cycles after repair.