The 2 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 # 1606335
If the refrigerator is not defrosting, the defrost timer might be defective. Several times a day the defrost timer is supposed to turn on the defrost heater to melt away any frost that may have accumulated on the freezer evaporator coils. If the timer is defective it won't advance and won't turn on the heater.
RepairClinic Item # 1514997
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.