Some defrost systems use a thermostat (a bi-metal switch) to prevent the defrost heater from overheating. The switch is normally closed. During a defrost cycle, the defrost heater causes the metal alloy in the switch to warm and as it does it curls back and breaks the circuit. As the metal cools, it makes a circuit again and the defrost heater starts heating again (as long as the defrost timer is in the defrost cycle). A defective thermostat can prevent the defrost heater from coming on or allow it to overheat which could result in heat damage or fire.
Before testing the defrost thermostat, unplug the refrigerator to avoid an electrical shock hazard.
The defrost thermostat is located near the defrost heater and is wired in series. It is usually located at the back of a side by side freezer or under the floor of a top freezer. It will be necessary to remove obstructions such as the contents of the freezer, freezer shelves, icemaker and the inside rear or bottom panel of the freezer.
The panel may be held in place by retainer clips or screws. Remove the screws or depress the retainer clips with a small screwdriver. On some older top freezers it is necessary to remove the plastic molding to access the freezer floor. Removal of that molding can be tricky -never force it. If you decide to remove it, you do so at your own risk - it is prone to breaking. Try warming it first with a warm, wet towel.