How to Replace a Defrost Thermostat in a Refrigerator

Diagnose the Problem

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.

Defrost Timer

Defrost Timer Located Near the Controls
Defrost Timer Located Near the Controls

Test the Defrost Timer

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.

Inspect the Heater

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.

Defrost Heater

Replace the Defrost Thermostat

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.

Remove Back Panel

Back Panel Removed

Freezer section with back panel removed.
Freezer section with back panel removed.
Locate the Thermostat and Remove
Locate the Thermostat and Remove

Defrosted Coils Wiring Harness Removed

Ice Defrosted
Ice Defrosted

Remove the Thermostat

Wiring Harness Removed From Freezer
Wiring Harness Removed From Freezer
Defrost Thermostat cut off of the Wiring Harness
Defrost Thermostat cut off of the Wiring Harness

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Comments 16 comments

Wesman Todd Shaw profile image

Wesman Todd Shaw 6 years ago from Kaufman, Texas

Holy smokes! You'd never in a million years do better with the pictures!

I'm thinking, "he took those pics himself."

I used to be in refrigeration-working on bun machines-the chipped ice kind, for slurpee style drinks, etc.

Mostly, I just do hvac. I've got an interesting job coming up, and I'm going to publish a hub about the product. . . .Ambiance Eco Green mini split heat pumps.

I've got a customer with four of them on a house(smart air conditioning and heating!). . . .and I did the start up, but not the install. Anyways, one of them has got to have either a bad thermostatic expansion valve, or is completely stopped up at a screen protecting an orifice somewhere. It's got a full factory charge(410a). . .but when the compressor kicks on(and it's a two speed with freq drive fan motors in and out). . . .my suction pressure goes to zip.

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest Author

@Wesman Todd Shaw You better believe I took the photos, I just replaced the defrost thermostat in my refrigerator and took advantage of the job.

Those were taken with my little point and click, and then I was able to use Picasa to add the labels. It is so simple.

I have fixed a hundred refrigerators, but know very little about heating and ac. Sounds like you have a job on your hands. Be sure to take lots of photos, I took about 100 and chose those to show. I want my hubs to lead to someone being able to make a repair themselves.

Thanks for stopping by and for the cool comment.

Wesman Todd Shaw profile image

Wesman Todd Shaw 6 years ago from Kaufman, Texas

Thanks SteveoMc, I'm rather broke. . . only net pics for me right this minute. BTW, on this game we play called "traffic," I used StumbleUpon to rate you and this one as "awesome appliance repairman!"

I'm going to see if I can generate some traffic for myself that way too.

I like this kind of work, really-I enjoy it. . . when I know what I'm doing and get it done right, it's a good feeling, and pays okay too.

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest Author

@Wesman Todd Shaw Thanks for the Stumble action, I appreciate it. I get a few views that way, it always works better when someone else stumbles you. My son called me to say the other day that he was stumbling and randomly came up with one of my hubs. That was cool. I don't know where you get your pics, but I get some really good ones from all free, no strings. Don't even have to give attributes. I have a lot of them in my hubs.

Once again, I appreciate your support.

justom profile image

justom 6 years ago from 41042

You're the man Steveo! I was thinking about you a few days ago when the exaust fan in the bathroom crapped out. I took off the cover and it was so dirty. I gave it a good cleaning and it's now doin' fine. I'm telling you, your hubs are gonna' be used sooner or later. Great hub, I hope you're doing well. Peace!! Tom

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest Author

@justom Thanks once again! Love seeing you come by and I hope you are right about the hubs. I just try to keep fixing my stuff. LOL

I am doing well btw, thanks.

BJBenson profile image

BJBenson 6 years ago from USA

Great hub. I think those picture have blown all of us away!

What would we do without you?

Hope we never find out!

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest Author

@BJBenson Where the heck have you been? Glad to see you back, I wondered about it. Thanks for picking me up again, great to see you and thanks for the comment, nice.

Hello, hello, profile image

Hello, hello, 6 years ago from London, UK

I can only join in the chorus of praise for your pictures. Thanks for the brilliant hub.

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest Author

@Hello,Hello Thanks once again for your continued support....glad you came by.

danatheteacher profile image

danatheteacher 6 years ago from Pacific Northwest

Steve, ur just a stud when it comes to this subject!

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest Author

@danatheteacher Haven't been called a stud since 1980, thanks for the youthful infusion. Glad you came by.

whitton profile image

whitton 6 years ago

This is a really a great Hub. Nice steps and photos of how to replace a defrost thermostat in a refrigerator.

whitton profile image

whitton 6 years ago

Pixielugs 5 years ago

Very nice - have you also considered doing one about replacing the defrost heater?

Didge profile image

Didge 4 years ago from Southern England

Good one SteveoMc :)

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