Refrigerator or freezer temperature fluctuates; defrost seems irregular; E4 displayed on control panel.

Safety first: Disconnect mains power. Be cautious of sharp evaporator fins and edges.

Procedure:

• 1. Defrost the unit: If there is heavy ice on the rear panel, unplug the refrigerator and allow it to defrost completely (up to 24 hours). This can temporarily restore function if ice was affecting the sensor.
• 2. Access the evaporator: Remove freezer drawers and the inner rear panel. The evaporator sensor is usually clipped to the evaporator tubing or mounted in a small well on the coil.
• 3. Inspect sensor placement: Ensure the sensor is firmly clipped to the correct location on the evaporator. A loose sensor can misread temperature.
• 4. Test the sensor: Disconnect the sensor and measure resistance at the current temperature (use an infrared thermometer if available). Compare to Siemens NTC chart. Replace if open, shorted, or out of tolerance.
• 5. Check wiring: Follow the sensor wires back to the harness connector. Look for brittle insulation, cuts, or stretched sections. Repair or replace as needed.
• 6. Reassemble and test: Refit the rear panel and drawers, restore power, and monitor for recurrence of E4. Persistent E4 with a known-good sensor suggests a main PCB fault.

Warning: Do not use knives or screwdrivers to chip ice from the evaporator; this can puncture refrigerant lines and render the appliance non-repairable.