Freezer display shows E4 and unit may not defrost, or defrost heater never energizes.

Safety warning: Unplug the freezer before accessing the evaporator or control board. The defrost heater operates at mains voltage and can cause electric shock.

Steps to diagnose E4:

• Manually defrost first: Unplug the unit and allow all ice on the evaporator to melt. This prevents damage to panels and makes components accessible.
• Access the evaporator and heater: Remove baskets and the rear interior panel. The defrost heater is usually a metal or glass-encased element mounted under or around the evaporator coil.
• Visual inspection: Look for burnt spots, broken heater tubing, or disconnected wires. Check the defrost thermostat/thermal fuse (small round or inline device clipped to the evaporator tubing).
• Test the heater: Disconnect heater leads and measure resistance with a multimeter. A typical heater will show some resistance (e.g., 20–200 Ω). OL (open) indicates a failed heater.
• Test the defrost thermostat/fuse: At room temperature, many defrost thermostats may read open; test again when cold (place in ice water). A thermal fuse should read continuity at all times; OL means it is blown.
• Check wiring to control board: Inspect the harness from the heater/thermostat to the main control board for burns or loose connectors.
• Replace faulty parts: Install a compatible Costway defrost heater and/or defrost thermostat/thermal fuse as needed. Ensure all connections are tight and properly insulated.
• Reassemble and test: Reinstall the interior panel, plug in the freezer, and allow it to run. The E4 code should clear after a successful defrost cycle.

Note: If heater and thermostat test good, the defrost relay or control board may be defective and should be evaluated by a professional.