Thick ice buildup on the evaporator coil behind the rear interior panel.

Safety first: Unplug the freezer before removing panels and working near the evaporator.

• Manual defrost: Remove all food and shelves, then remove the rear interior panel. Allow the ice to melt completely or use a hair dryer on a low setting to speed up the process.
• Test defrost heater: Once exposed, disconnect the heater leads and check for continuity. Replace the heater if it shows an open circuit.
• Check defrost thermostat: When the evaporator is cold, the thermostat should show continuity. Replace it if it remains open.
• Inspect thermistor: Measure its resistance and compare it to the manufacturer's specifications. Replace it if it is out of range.
• Control board: If all components test correctly, the main control board may be failing to initiate defrost cycles and should be replaced.