How to Test the Defrost Heater and Thermostat in a Refrigerator


Frost-free refrigerators have an automatic defrost system, consisting of a defrost heater, defrost thermostat and a defrost timer. The timer sends power to the defrost heater every eight to 12 hours, depending on your refrigerator model, which turns on the heater, melting ice and frost that has developed on the evaporator coils. Between the timer and heater is a thermostat, which expands as it warms and shrinks as it cools. The ice is enough to cool the thermostat, which, when it shrinks, allows power to pass through it to the heater, which then turns on. The thermostat allows the heater to run just long enough to melt the ice, then turns it off. If the timer is functioning properly but ice is still developing on the coils, test the heater and thermostat using a multimeter to determine which one has gone bad.

Things You'll Need

  • Screwdriver
  • Multimeter
  • Wire clippers
  • Wire strippers
  • Wire nuts
  • Silicone sealant
  • Unplug the refrigerator from the wall outlet.

  • Set the multimeter to the lowest setting designed to measure ohms of resistance, then calibrate the meter. For specific instructions, refer to the meter's manual.

  • Remove the contents of the freezer, including the freezer shelves.

  • Remove the air duct if applicable. To remove the air duct, take out the screws at the bottom of the duct, then lift the duct up and out.

  • Take out the rear panel of the freezer by removing the four screws securing the panel to the freezer. Lift the panel out.

  • Locate the defrost thermostat, which is clipped to the top of the evaporator coils. Ensure that the coils are frozen over prior to testing, as a working thermostat will appear to be nonfunctional when it is warm.

  • Cut the two wires going to the defrost thermostat, about 2 inches from the thermostat. Do not remove the thermostat from the coils.

  • Strip approximately 1/4 inch of insulation off the wires connected to the thermostat.

  • Touch one of the bare wires with one of the multimeter probes. Touch the other wire with the other probe. The display will indicate an infinite number of ohms of resistance if the thermostat is bad, and it will move significantly if the thermostat is functioning properly.

  • Remove the defrost heater strap and mounting bracket from the underside of the evaporator coils if the thermostat is found to be functioning properly.

  • Bend the tabs under the coils out of the way, then pull down the heating element.

  • Pull the wires off both the heating element terminals.

  • Touch one terminal with one probe of the multimeter, then touch the other terminal with the other probe. The display will indicate an infinite number of ohms of resistance if the heating element is bad, and it will move significantly if the heating element is functioning properly.

Tips & Warnings

  • To reconnect the thermostat, strip the other two wires, then reconnect the wires using wire nuts. Fill the wire nuts with silicone sealant to strengthen the connection.

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