The part(s) or condition(s) listed below for the symptom Refrigerator not defrosting are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The defrost heater assembly turns on several times throughout the day to melt away any frost that may have accumulated on the evaporator coils. If the defrost heater assembly is defective, the defrost system will not work, and frost will continue to accumulate on the evaporator coils. To determine if the defrost heater is defective, use a multimeter to test it for continuity. If the defrost heater does not have continuity, replace it.
The main control board regulates the power supply to all of the refrigerator components, including the defrost system. If the main control board is defective, it may stop sending voltage to the defrost system. Control boards are frequently misdiagnosed—before replacing the main control, first test all of the defrost system components. If none of the defrost system components are defective, consider replacing the main control board.