Refrigerator shows E4 and there is heavy frost or ice buildup on the freezer back wall

Safety first: Unplug the refrigerator. Water from defrosting can create slip and shock hazards; protect the floor and keep cords away from water.

Steps to check:

• Manual defrost: Open doors and allow all ice on the evaporator and back wall to melt completely. This may take several hours. Do not use knives or screwdrivers to chip ice.
• Access defrost components: Remove the freezer rear panel to expose the evaporator, defrost heater (usually at the bottom or woven through the coil), and defrost thermostat (clipped to the coil).
• Test the heater: Disconnect heater leads and measure resistance. A typical heater will show a finite resistance (e.g., 20–150 Ω). Infinite resistance indicates an open heater that must be replaced.
• Test the defrost thermostat: While it is cold (below freezing), it should read near 0 Ω (closed). At room temperature, many types open (infinite Ω). If it never closes when cold, replace it.
• Inspect wiring: Look for burnt connectors or brittle wires. Repair or replace damaged sections with high-temperature-rated wire.

Note: If heater and thermostat test good, the main control board may not be initiating defrost and may require replacement.