How to Tell if Your Car's Thermostat Is Broken & What to Do to Fix It

Photo: DisobeyArt (Shutterstock)

When a car’s heat is working, it can feel glorious on a cold day—thawing you out so efficiently that you might have to take your coat off to keep from overheating. (Just make sure you don’t do that while you’re driving.) And if you’re lucky enough to have heated seats? Basically a trip to the spa. (Although instead of sipping on cucumber-infused water, it’s a half-empty water bottle that has been sitting in your car for who-knows-how-long.)

So when your heat isn’t working, you notice. In addition to dealing with frosted and foggy windows, it’s another one of the “fun” parts about driving in the winter (in cold climates). There are several different reasons why your heat won’t come on (Jonathon Klein goes over seven of them in this article in The Drive), including the most common cause of your lack of heat: a broken thermostat. Here’s how to figure out if yours is broken, and if it is, how to fix it.

How to tell if a thermostat is broken

While a faulty thermostat isn’t the only reason your heat isn’t working, Klein says that it’s the one that’s most frequently to blame. “Stuck open or stuck closed, the part can not only cause issues with your heat but also your engine’s cooling system,” he writes.

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Aside from the overall lack of heat, here are a few other clues that your thermostat might be broken, courtesy of YourMechanic:

• The temperature gauge is reading really high (in the red) within the first 15 minutes of starting the vehicle.
• The engine has been sitting for a while and is not warm.
• The vehicle’s temperature is changing erratically.
• There is coolant leaking from around the thermostat housing or under the vehicle.

How to fix a broken thermostat

Fortunately, this is a relatively straightforward job you can likely do yourself, and Klein provides a set of easy-to-follow instructions.

First up is safety: he recommends safety glasses and mechanic’s gloves. Now, here’s what else you’ll need:

• Drain bucket
• Selection of wrenches
• Coolant
• New thermostat
• A flat workspace (like a garage floor or driveway)

1. Let the car cool for 15 to 20 minutes.
2. Locate the thermostat. It will be at the base of the radiator, in between the core and the main hose.
3. Remove the radiator cap.
4. For better clearance, lift up the front end of the vehicle.
5. Place a bucket underneath the radiator and drain the coolant by detaching the hose.
6. Remove and replace the thermostat.
7. Reattach the hose to the radiator.
8. Add the coolant and place the cap back on the reservoir.
9. Lower your car.
10. Start the engine.
11. Wait to see if the heat comes on.
12. Take a test drive.
13. Check to make sure the coolant level hasn’t dropped.
14. If it has, refill it as necessary.

Now, enjoy your warm and toasty ride.

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