The parts or conditions listed below for the symptom Refrigerator not defrosting are ordered from most likely to least likely to occur.
The defrost control board determines how often the defrost cycle is run. If the board is faulty, the refrigerator will not enter the defrost cycle. Before replacing the defrost control board, make sure the heater and the defrosting thermostat are functioning correctly. If they are, it’s very likely that the defrost control board is faulty and needs replacement.
The defrost timer turns on the heater several times during the day to melt away any frost accumulating in the evaporator coils. If the timer is faulty, it is possible that the defrost cycle won’t start or that no energy is sent to the heater. To determine if the defrost timer is faulty, slowly advance the dial to the defrosting cycle; compressor must turn off and heater must turn on. If no energy is sent to the defrost components, or it advances out of the defrost cycle, it’s a good indicator that the defrost timer is faulty and need replacement.
The heater is switched on several times a day in order to melt any frost accumulating in the evaporator coils. If the defrost heater set is faulty, defrost system will not run and frost will continue to accumulate in the evaporator coils. To determine if the heater is faulty, use a multimeter to test for continuity. If the defrost heater has no continuity, replace it.
The defrost thermostat, controls the temperature on the evaporator coils. When temperature on the evaporator coils falls below the set temperature, thermostat contacts are closed allowing energy to enter the defrost heater during the defrost cycle. During the defrost cycle, the heater melts any frost accumulating in the defrost coils. If the defrost timer is faulty, contacts on the thermostat will not close and energy will not get to the heater. To determine if the defrost thermostat is faulty, use a multimeter. If the thermostat has no continuity when it reaches below operation temperature, replace it.
The defrost sensor records temperature on the evaporator. The sensor has a backup fuse in case the sensor fails. If the defrost sensor does not turn off the heater the fuse will melt turning of the heater. If the fuse inside the sensor has melted, the defrost system will not work, and the fridge will accumulate frost. The fuse on the sensor cannot be replaced; if the fuse is melt; the sensor must be replaced. To determine if the defrost sensor or the sensor’s fuse are faulty, use a multimeter to test for continuity. If the sensor or the fuse have no continuity, replace the defrost sensor.
The main control board regulates energy input for all components on the refrigerator; includen elements of the defrost system. If the main control board is faulty; it may stop sending energy to the defrost system components. This however is rare, main control boards are often misdiagnosed as faulty; before replacing the main control board, test all components of the defrost system, if none of them is faulty; consider replacing the main control board.
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