Freezer displays E4 and may not defrost; ice accumulates on the evaporator, fan noise increases, and cooling performance drops.

Safety warning: Always disconnect mains power before testing heater circuits. The defrost heater operates at full line voltage and can cause electric shock.

Diagnostic procedure:

• 1. Defrost and access evaporator: Allow ice to melt, then remove the interior rear panel to expose the evaporator and heater.
• 2. Locate the defrost heater: It is usually a metal or glass-encased element running along the bottom or woven through the evaporator coil.
• 3. Visual inspection: Look for burn marks, broken heater tubes, or disconnected wires. Check connectors for signs of overheating.
• 4. Test heater resistance: Disconnect the heater leads and measure resistance with a multimeter. Typical values are between 50–300 Ω depending on the model. An open circuit (infinite resistance) indicates a failed heater.
• 5. Check wiring to control board: Inspect the harness from the heater to the main PCB. Repair any burnt or brittle wires and ensure connectors are tight.
• 6. Evaluate defrost relay: If the heater tests good, the control board may not be energizing it. With the unit powered and extreme caution, a technician can test for voltage at the heater during a forced defrost cycle. If no voltage is present and wiring is intact, the board/relay is likely faulty.
• 7. Replace components: Install a new Bomann-compatible defrost heater if defective. If the board is faulty, replace the main control board.

Note: After repair, monitor the freezer for several days to ensure frost does not re-accumulate on the evaporator.