Refrigerator shows F3 and may not defrost properly; ice buildup on rear wall or evaporator.

Safety: Unplug the refrigerator. The defrost heater operates at mains voltage and can cause electric shock.

Diagnostic steps:

• Defrost manually first: Open doors and allow the unit to defrost completely (this may take several hours). Place towels to catch water. This will allow you to access the evaporator and heater.
• Access heater: Remove the interior rear panel in the iced‑up compartment. The defrost heater is typically a metal or glass‑encased element at the base or woven through the evaporator.
• Visual inspection: Look for burn marks, broken heater tubing, or melted connectors.
• Continuity test: Disconnect heater leads and measure resistance with a multimeter. A reading of infinite resistance indicates an open heater that must be replaced. Also test any inline thermal fuse or thermostat for continuity.
• Check connectors: Inspect and clean any oxidized or burned spade connectors; replace if heat‑damaged.

Note: If heater and safety devices test good, the fault may be in the main control board's defrost relay or triac, which should be diagnosed and replaced by a Miele technician.