How to Unstick a Thermostat in a Car

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Close up of the car engine coolant thermostat

When your car’s thermostat is stuck open or closed, there are a series of nasty things that can happen. The worst potential issue is an engine that overheats and catastrophically fails. To avoid this, you should go ahead and unstick your thermostat.

Rather than trying a DIY method to unstick the thermostat in your car, you should simply replace the part. It’s inexpensive and a fairly straightforward process. In short, you need to drain your coolant, disconnect the housing, remove the current thermostat, put in a new one, reconnect the housing, then refill your coolant line.

In this guide, I’ll tell you exactly how to unstick the thermostat in your car. I’ll also walk you through the troubleshooting process and give you an in-depth, step-by-step guide to replacing a faulty thermostat in your car.

What Does the Thermostat in a Car Do?

Thermostats come in various forms. The thermostat of your car serves a single purpose: to regulate the temperature of your engine. Unlike your home’s thermostat, you can’t adjust this temperature at all — it’s determined by the auto manufacturer and the target temperature is within the perfect operating temperature of your engine.

The thermostat works like a drain plug in your bathtub. It will open and close, allowing or preventing fluid to flow through the radiator.

Engine Cooling System Illustration
Engine Cooling System

The fluid in question is coolant. This coolant is really good at absorbing energy and moving it away. It gets used in your engine to basically “suck up” the heat in your engine and transfer it somewhere else, effectively cooling down your engine.

When the thermostat is stuck, it can either be overcooling your engine or not cooling it at all.

The Problem with a Stuck Thermostat

Whether a thermostat is stuck open or closed, there are a number of risks and problems to be aware of. Let me take a second to briefly explain the issues behind either of these conditions.

Engine coolant thermostat close up isolated against a dark background

A Thermostat That’s Stuck Closed

If the thermostat is stuck closed, that’s when serious problems can occur. In this case, your engine doesn’t have any assistance in cooling down. The engine will keep getting hotter, warming up the nearby coolant, and overheating.

In a stuck closed condition, your engine can smoke, blow your head gasket, and critically damage the valves.

A Thermostat That’s Stuck Open

When the thermostat is stuck open, that means that coolant is continually flowing. This might not seem like a problem, but it can still affect the ideal temperature of your engine.

In this scenario, the engine will take too long to warm up. The ideal temperature range obviously has an upper limit (this is when things smoke and blow up), but it also has a lower limit. Engines don’t like to run cold — that’s why people idle their cars during the winter months.

A mechanic inspecting the engine coolant thermostat by removing it

With a thermostat that’s stuck open, you’ll notice a significant performance drop. In addition, you’ll probably be fueling up more often since the fuel economy will go down.

Your car’s heater will probably not work, and the temperature gauge will be far below a normal level.

How to Tell That the Thermostat Is Stuck

Before unsticking your thermostat, you need to make sure it’s actually stuck in the first place. Here are a few quick ways to see how your thermostat is doing.

Look at the Coolant Flow

Since the thermostat regulates the flow of coolant, you should start by looking at the coolant. In cars, this coolant is a liquid that flows when the thermostat is open and sits stationary when it’s closed.

Open the hood of your car and look for the coolant cap near your radiator. The radiator is a thin box that will be either black or silver, near the grill of your car.

You shouldn’t touch the cap if you were recently driving your car. The coolant will be boiling hot and can seriously burn your skin when it splashes out. Wait for the car to fully cool before doing this troubleshooting step.

Car radiator cooling system with a person removing the radiator cap to inspect the coolant flow

Find the cap and loosen it then remove it. Make sure you keep this cap somewhere safe, so you don’t lose it.

Turn your car on and keep it idling. Stand in front of your car and take a look inside this coolant cap. After 10 to 20 minutes, if your thermostat is working properly, the coolant should start to flow.

If you see coolant moving immediately after starting your car, the thermostat could be stuck open. If it doesn’t move at all, it could be stuck closed.

Take a Look at the Engine Temperature Gauge

A quick indication that something is wrong with your thermostat is the nature of your gauge. If the needle is way lower or higher than where it typically sits, then something is wrong.

Car temperature gauge overheating 2
Car temperature gauge showing that the engine is overheating

When you first start your car, the gauge will be very low. Ignore this. Instead, turn your car on and drive it around for 10 to 15 minutes before looking at the gauge.

If the needle is near the “C” or “H”, then you have a problem. Although it isn’t a guarantee that the thermostat is stuck in this case, it’s a good place to start troubleshooting.

Grab a Temperature Measurement

Another good indicator is the temperature under the hood. Since the thermostat is there to keep things at a perfect temperature, any deviation should raise a red flag.

A mechanic using an infrared heat gun thermometer to inspect the temperature of the cooling system

If you have an infrared temperature gun, this is the time to use it. With your car running, take two initial temperature readings: one on the engine itself, and one on the hose on the upper portion of your radiator. This part of the hose occurs right before the coolant is fed into your engine.

If these two readings are within 2 degrees F, then you probably have a thermostat that’s stuck open.

Etekcity Infrared Thermometer 774, Digital Temperature Gun, Non-Contact Electric Laser IR Temp Gauge

Etekcity Infrared Thermometer 774, Digital Temperature Gun for Cooking, Non Contact Electric Laser IR Temp Gauge
Etekcity Infrared Thermometer

Wait 15 minutes and take another pair of temperature readings from these two spots.

If both areas are the same temperature but your engine is running hot, then a stuck thermostat isn’t your problem. If the hose is a lot cooler, then you have a thermostat that’s stuck closed.

Ideally, both readings will be within 10 degrees F of one another, and your temperature gauge is right in the middle.

HVAC Isn’t Working Right

If too much coolant is being used through your radiator, then there won’t be enough to change the incoming air.

In this case, the HVAC in your car will either not be able to blow hot or cold air. The telltale sign that it’s related to the thermostat is if only one of the two temperatures works for your HVAC (in other words, just the heater or just the AC).

A person toggling adjusting the knob on a temperature climate control switch in the car

Before blaming the thermostat, take a look at your HVAC system and diagnose it. If the HVAC system doesn’t have anything broken in it and you don’t see any coolant leaks, then your thermostat could be causing this issue.

You’re Fueling Up More Often

When a thermostat is stuck open, then your engine has to work harder to generate power. When this happens, you’ll burn through fuel much quicker.

If you’re suddenly losing 5 mpg in your vehicle, then start questioning your thermostat. There’s a good chance that it’s stuck.

Take it Out and Check

The best way to know if your thermostat is stuck is to take it out and closely examine it. The process doesn’t take a lot of time and the thermostat is surprisingly easy to access. I outline this process step-by-step in a later section.

How to Unstick a Thermostat in a Car

Unsticking a thermostat isn’t as easy as people make it seem. Maybe I just have bad luck, but I’ve tried different methods a number of times in my past, and none of them have worked to reliably unstick a thermostat.

A thermostat can be jammed up due to debris buildup, a failing coil, rust, or degradation of the part. All of these issues are very hard to remedy.

Since thermostats are so inexpensive, it’s usually best to just replace the one that you have. Otherwise, you’ll have to go through a replacement process every time you try a new DIY solution to unstick it.

In short, I wouldn’t suggest trying to unstick the thermostat in your car, just replace it with a new one.

How to Replace a Stuck Thermostat in a Car

For a vast majority of thermostat troubleshooting and repairs, you’ll want to fully replace the piece. It’s inexpensive and easy to do. Follow these steps and you’ll have a new thermostat in your car in no time.

Step 1: Drain the Coolant

During this process, you’re going to be exposing your coolant lines. If coolant is still in your system, it will start pouring out and leaking all across your engine bay. That makes for an annoying cleanup and can damage parts along the way.

Lumax LX-1632 Black 15 Quart Drainmaster Oil Drain Pan

Lumax LX-1632 Black 15 Quart Drainmaster Oil Drain Pan
Lumax LX-1632 15-Quart Oil Drain Pan

To be smart, you should start by draining out all the coolant. Hop under your car or use a DIY car ramp for easier access. Locate the coolant line and find the bolt that plugs everything. It will be towards the front of your car at the bottom of your radiator.

Position a bucket under your car before you get started.

Take out the plug, and let the coolant lines fully drain. A lot of fluid is going to come out and will probably stream out, so be prepared to move the bucket and act fast during this.

Step 2: Remove the Housing

Locating the thermostat housing is pretty easy. Look towards the front of your car. There should be a small metal piece that has a number of tubes or hoses leading to it (typically 2) and an electrical wire. This is the housing.

To remove it, you’ll first need to remove all of the coolant hoses that connect to it. A flathead screwdriver can be used to remove the screw that’s fastening the worm-drive tube clamps. When the clamp is loosened, you can pull the hose straight back.

Repairing car radiator

Next, remove the electrical wire that’s running to the housing. Be careful with this and make sure you don’t cut any of the wires as you remove them.

With these components removed, you’ll need to remove one or two mounting bolts. These can be removed with a simple ratchet wrench and socket driver. One bolt will be a lot longer than the other, so remember which one goes where.

After the bolts are removed, the housing will pull off pretty easily. Make sure you put the housing, hose clamps, and all fasteners somewhere safe so you don’t lose them for the reinstall later.

Step 3: Remove the Thermostat

Now, the thermostat is fully exposed. After removing the housing, the thermostat is the metal piece that’s sitting in the hole.

Don’t touch it yet. First, take a picture of it so you remember the orientation. It’s possible (and honestly pretty easy) to put the thermostat upside-down, and you definitely don’t want to do that.

Mechanic removing the engine coolant thermostat from the vehicle

Try to pry off the piece with your fingernail, but you might need to use a flathead screwdriver to carefully pry it loosely. Be very careful as you handle this piece.

This is also a good opportunity to look at the status of your thermostat. There’s a spring that coils around, allowing the head of the thermostat to open and close under certain conditions.

If you try to move the sprung piece and there’s no motion, then the coil is probably jammed and you’re dealing with a stuck thermostat. If the head is fully depressed and the coil is tight, then your piece is stuck open.

Step 4: Clean the Interfacing Surfaces

Before reinstalling a thermostat, you need to make sure the mounting surfaces are both clean. It’s common for these areas to get greasy or grimy as you drive along.

With an interference between these surfaces, coolant can leak or moisture can make its way into the thermostat housing. This will cause rust which will make your thermostat get jammed again.

View of the lower part of the engine with parts removed ready for cleaning and new parts to be installed

In the past, I’ve used a razor blade to clean both surfaces, then a shop towel afterward with Simple Green on it. You can also use a small angle grinder with a thick pad on it for quicker results.

During this cleaning step, make sure you’re not slicing open any nearby wires or tubes. This will lead to big problems in the future, so be careful and work slowly.

Step 5: Put a New Gasket On

A gasket needs to be installed on the thermostat housing before you reinstall it. You’ll need to do some digging to get an exact match for your car, but they are often sold with replacement housings.

New car engine coolant thermostat with a rubber gasket isolated against a white background

If your housing is badly damaged, then you should also replace the whole housing in this step.

Make sure the mounting holes match up with the cutouts of the gasket before adhering it on. Push the gasket down firmly and give it a few minutes to set before continuing the installation.

Step 6: Install a New Thermostat

It’s finally time to put in a new thermostat. Refer to the photo you took earlier to remember which side gets inserted into the thermostat’s hole and which side is sitting outside of the hole.

A mechanic re-installing the engine coolant thermostat back on the engine

If you have a newer thermostat, you might find a small hole in the flange. It’s a good idea to rotate the piece and install the thermostat so the hole is at the top. This is a vent hole, and it’s better to draw in air from the top rather than the bottom, where the gasket is more restrictive.

The installation is as simple as popping the thermostat into its hole. It’s going to be free-spinning and might not seem snug. This is fine. The housing is going to draw it in and keep it secure.

Step 7: Reinstall the Housing

Speaking of the housing, it’s time to put that back on. Remember to use the correct bolts in the correct holes so it mounts securely.

When you install the housing, it’s possible for your thermostat to fall out and get knocked out of position. This is an easy fix, as long as you check before reassembling the tubes and electrical wires.

Hand-tighten the longer bolt just so the housing is in position and you can take your hands away. Take a second and look through one of the tube holes in the housing. Check on the thermostat and make sure it’s still in the right position before moving forward.

Car engine coolant thermostat housing close up isolated against a white background

If it looks okay, then continue tightening the two bolts and using a ratchet to torque it down a little. Don’t overdo it, because you don’t want to strip these bolts and risk the housing coming loose.

Push both tubes back onto the housing and slide down the hose clamps. Tighten them and make sure the hoses don’t budge when you pull on them.

Reattach the electrical cable and make sure it snaps or clicks into position.

Step 8: Refill Your Coolant

Time to refill the coolant that you drained in step 1. Start by filling it at the cap in the front of your car near the radiator. Completely fill this section, then fill up the reservoir in the rear corner of your engine bay. There’s a line that identifies how much coolant to put in the reservoir.

Topping of radiator engine coolant into the coolant reservoir

Conclusion

I hope that your car is running cool after going through my step-by-step guide earlier. I’ve personally used this process, so I know that it works. If you want to learn more about DIY fixes for your car, take a look at more installation and troubleshooting guides on my site. In addition, I have a list of car products that might make your life a little easier.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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