Display shows E4 and the cooler stops cooling or ices up heavily

Safety: Disconnect power before accessing the evaporator or control board. Beware of sharp fins and edges.

Steps to address E4:

• Check for frost buildup: Remove interior shelves and inspect the rear wall. A solid sheet of ice indicates a defrost problem.
• Manual defrost: Unplug the unit and leave the door open until all ice melts. Do not use sharp tools to chip ice; this can puncture refrigerant lines.
• Access evaporator components: Remove the rear interior panel to expose the evaporator coil, defrost heater, and defrost sensor.
• Test defrost sensor: Disconnect the defrost NTC sensor and measure resistance at room temperature and again in ice water. Resistance should change smoothly according to Bosch specs. Replace if open, shorted, or erratic.
• Test defrost heater: With the unit unplugged, disconnect heater leads and check resistance. A typical heater will show a finite resistance (not open). If open, replace the defrost heater.
• Inspect wiring and connectors: Look for burnt connectors or broken wires between the heater, sensor, and control board.
• Control board evaluation: If both heater and sensor test good, the defrost relay or control logic on the main PCB may be faulty and require replacement.