How to Replace a Defrost Thermostat in a Refrigerator

Start by following some general safety rules.  Unplug the refrigerator while you are working on it.   Click here to follow the safety rules.  Do not chip, scrape, or pry the ice, severe damage or serious injury could result.

There are generally 3 components to the defrost cycle: a timer, a thermostat, and a heater. If any of these three components fails, then the refrigerator will stop cooling. The entire cold in the box is dependent on air circulation and the correct operation of the defrost cycle.

If the refrigerator defrost cycle does not operate correctly, ice will build up under the back panel of the freezer and will block the air flow. Without proper air flow, the refrigerator will not cool. It will begin to warm in the refrigerator section.

The problem is difficult to diagnose, since the defrost thermostat shows no continuity until is has reached a low temperature usually freezing or below. If you take the thermostat out of the freezer, then it will warm rapidly and you will be unable to test it. It will be too warm.

The timer is more difficult to test since it has several leads and a timer that advances.

The heater is easiest component to test, it will show continuity if good and 100% resistance is bad.

To test the defrost timer, advance the timer until the refrigerator turns off.  Leave it in this position until it turns itself back on, this will take around 20 minutes or so.  

If it turns the refrigerator back on, then chances are that the timer is good.   At least we know that it turns the refrigerator off and on. 

The timer can be located in several places, the most likely place is near the cold control.   If it is mechanical, then it will have an exposed button shaped knob that can be turned with a screwdriver.    It is notched so that the timer cannot be turned backward and it usually operates in a clockwise fashion.    A second likely location for a timer is in the back of the refrigerator near the compressor.   Search for an opening that  has a timer knob set inside.

Some refrigerators locate the timer in the front just behind the kick plate or grill in front of the coils.   Take the kick plate off and examine the area for a timer.

Visually inspect the heater, and determine if it is a heating element like the one in your stove, a calrod that heats up, or if it is a coil enclosed in glass. If it is a solid element, they are pretty stable and will be less likely to be the problem.

Do a continuity test on the heater.  If it shows continuity, then the heater is good.  I the heater is bad, then it has to be replaced.

That leaves the thermostat.

You have determined that the heater is good and the defrost timer is good.  

A defrost thermostat is hard to test, they usually operate when they are cold.  Up to  50 degrees or so they will show continuity, above 50 degrees they will show no continuity.  It is difficult to test them because when you take them out of the freezer, they will be above 50 degrees.   If you have a separate freezer, put it in the freezer for an hour and then test it, if it has continuity at low temperatures, then is might be good.  If you take it out of the refrigerator then it is best to replace it, it only runs about $10.

You have determined that the defrost thermostat is bad.   Now you need to replace it.  In some refrigerators, it is part of the wiring harness.   In others it has connectors.

Here are the steps:

1. Remove the back panel of the freezer.

2.  Locate and remove the thermostat.   Use caution here as the wiring harness is delicate in some respects and you should not pull and force the connections.  Most of them have a clip or plastic retainer that has to be depressed or pried in order to release it.  

3.  Let the evaporator coils defrost completely, do not pry or pick at the ice.  You may damage the refrigerator or injure yourself.  Do not smoke or have an open flame while working on the refrigerator, the gasses that could be accidentally released turn poisonous with open flame and could result in severe injury or death.   Never use a sharp object or scrape the coils.

To speed the defrost process, you may use warm water and a turkey baster to put water directly on the ice, the water will pool below and will need to be soaked up with towels.

4.  A defrost thermostat that is wired directly to a wiring harness may be removed by cutting the wires close to the thermostat.   The new one then can be spliced into the wires.  Use silicone sealant to seal the wire nuts and to prevent moisture from infiltrating the splice.

5.  Replace the wiring harness, and connect the thermostat to the coil as it originally was connected.  

6. Replace the back cover of the freezer.  You are done.