The 3 most common part(s) or condition(s) which cause the symptom Refrigerator not defrosting are listed below starting with the most likely. Check or test each item and watch any available videos.
RepairClinic Item # 401456
If the refrigerator is not defrosting, the defrost control board might be defective. The defrost control board monitors several conditions and determines how often to run the defrost cycle. If the board fails, the refrigerator will not go into the defrost cycle. Eventually, ice will build up on the evaporator coil and then the refrigerator and or freezer will not stay cold enough. This will depend on where the evaporator is located. It's prudent to check the defrost heater and defrost thermostat for continuity before replacing the defrost control board.
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RepairClinic Item # 387642
If the refrigerator is not defrosting the defrost heater assembly might be burned out. Several times a day the defrost heater assembly turns on for a few minutes to melt away any frost that may have accumulated on the freezer or refrigerator evaporator coils. If the defrost heater assembly has burned out, the frost will build up on the coils and eventually no air will be able to travel through the coils to provide cooling. The defrost heater can be checked for continuity. If it doesn't have continuity it should be replaced.
RepairClinic Item # 379484
If the refrigerator is not defrosting, the defrost thermostat might be defective. Several times a day the defrost heater turns on for a few minutes to melt away any frost that may have accumulated on the freezer or refrigerator evaporator coils. In order for this to work, the defrost thermostat has to sense that the coils are cold enough. If the coils are cold enough the defrost heater will turn on. If the thermostat is defective it may not be able to sense the temperature of the coils and then won't turn on the defrost heater. If the defrost heater doesn't turn on, the frost will build up on the coils and eventually no air will be able to travel through the coils to provide cooling . The defrost thermostat can be checked for continuity. It should have continuity when it reaches the low temperature of its operating range.