How To Test the Defrost Thermostat
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.
The thermostat is connected by two wires. The wires are connected with slip on connectors or a wiring harness. Firmly pull the connectors or harness off of the terminals (do not pull on the wire). You may need to use a pair of needle-nose pliers to remove the connectors. Inspect the connectors and the terminals for corrosion. If the connectors are corroded they should be replaced.
Test the thermostat for continuity using a multitester. Set the multitester to the ohms setting X1. Place a probe on each terminal. The multitester should display a reading of zero when the thermostat is cold and a reading of infinity when it is warm (40 to 90 degrees F depending on the model). If the thermostat does not pass this test it should be replaced.