What a thermostat does and why it matters
Your car's thermostat is a valve that opens and closes to control how much coolant flows through your engine. When the engine is cold, the thermostat stays closed so the engine warms up faster. Once the engine reaches its normal operating temperature—usually around 195 to 220 degrees Fahrenheit depending on your car—the thermostat opens to let coolant circulate and prevent overheating.
A failing thermostat is one of the most common cooling system problems. When it gets stuck closed, your engine overheats. When it gets stuck open, your engine runs too cold, which wastes fuel and can damage the engine over time. Checking your thermostat is part of understanding whether your cooling system is working the way it should.
Key Takeaways
- You cannot see or test a thermostat without removing it from the engine, so diagnosis usually starts by watching temperature gauge behavior and checking coolant level.
- A thermostat stuck closed causes the temperature gauge to climb into the red zone within a few minutes of driving; a stuck-open thermostat keeps the gauge low even after the engine has run for several minutes.
- The thermostat housing is bolted to the engine block and connected to the upper radiator hose; you will need basic hand tools and a drain pan to access it safely.
- Testing a thermostat at home requires removing it and submerging it in hot water to watch whether the valve opens and closes at the correct temperature.
- If you are uncomfortable working inside the engine bay or draining coolant, a mechanic can diagnose and replace a thermostat in under an hour.
Signs your thermostat may be failing
Before you remove anything, watch how your car behaves. Turn on the engine and let it idle for a few minutes while you watch the temperature gauge on your dashboard. The needle should start low and gradually climb toward the middle of the gauge as the engine warms. Once it reaches the middle, it should stay there and not climb higher.
If the temperature gauge shoots into the red zone within the first few minutes of driving, your thermostat is likely stuck closed and coolant cannot circulate. If the gauge stays below the middle mark even after 10 or 15 minutes of highway driving, the thermostat may be stuck open. Either condition means the thermostat needs replacement.
Other signs include your engine running rough when cold, your heater blowing cold air even after the engine has warmed, or white steam coming from under the hood. Check your coolant level first—low coolant can mimic a thermostat problem. Open the radiator cap only when the engine is completely cold, and look at the coolant level inside the radiator or in the overflow tank.
Locating the thermostat housing on your engine
The thermostat lives inside a housing bolted to the engine block. The upper radiator hose connects to this housing and carries hot coolant from the engine to the radiator. To find it, follow the upper radiator hose from the radiator back toward the engine—it ends at the thermostat housing.
The housing is usually a metal casting about the size of a fist, with a bolt or two holding it in place. The exact location varies by engine type and car model. Your owner's manual or a repair manual specific to your car will show you the exact spot. If you are unsure, a quick online search for your car's year, make, and model plus "thermostat location" will show you photos or diagrams.
Before you touch anything, make sure the engine is completely cold. Hot coolant can cause severe burns. If you have just driven the car, wait at least 30 minutes before starting work.
Draining coolant safely before removal
You do not have to drain the entire cooling system to remove a thermostat, but you do need to drain enough coolant so it does not spill when you unbolt the housing. Place a drain pan under the thermostat housing. Loosen the drain plug on the bottom of the radiator or the lowest point of the cooling system and let coolant drain into the pan until the level drops below the thermostat housing.
Coolant is toxic and can harm pets and the environment. Never pour it down a storm drain or into the ground. Store used coolant in a sealed container and take it to an auto parts store, service station, or recycling center that accepts it. Many accept it for free.
Once enough coolant has drained, close the drain plug and move on to unbolting the housing. You will need a socket wrench or box wrench that fits the bolts—usually 10mm or 13mm. Remove the bolts slowly and set them aside where you will not lose them.
Removing and inspecting the thermostat
With the bolts removed, gently pull the housing away from the engine block. Coolant may still drip, so keep the drain pan underneath. Inside the housing, you will see the thermostat—a cylindrical or disc-shaped valve with a spring attached to the back.
Note which direction the thermostat faces before you remove it. The spring end usually points toward the engine block, and the valve end points toward the radiator hose. Take a photo with your phone if it helps you remember. Carefully lift the thermostat out of the housing and set it aside.
Look at the thermostat for obvious damage: cracks, corrosion, or a valve that appears stuck in one position. A thermostat that looks physically damaged should be replaced. If it looks intact, you can test it to see whether it opens and closes at the correct temperature.
Testing the thermostat with hot water
To test whether a thermostat opens and closes correctly, you need hot water and a way to measure temperature. Fill a pot with water and heat it on a stove. Use a thermometer—a cooking thermometer works—to monitor the water temperature as it heats.
Place the thermostat in the water and watch what happens. Most car thermostats begin to open around 180 to 195 degrees Fahrenheit and are fully open by 210 to 220 degrees. As the water heats, the valve inside the thermostat should gradually open. You will see the valve move or feel it loosen if you hold it gently. When you remove the thermostat from the hot water and let it cool, the valve should close again.
If the valve does not move at any temperature, or if it opens at a temperature far below or above the range for your car, the thermostat is faulty and should be replaced. If the valve opens and closes smoothly within the correct range, the thermostat itself is probably fine—your overheating or running-cold problem may be caused by something else, like a failing water pump or a clogged radiator.
Reinstalling or replacing the thermostat
If you tested the thermostat and it works correctly, you can reinstall it in the same direction it came out. If it failed the test or was visibly damaged, buy a replacement thermostat designed for your car's year, make, and model. Auto parts stores stock them for most vehicles, and they usually cost between $15 and $50 for the part alone.
Before you reinstall, clean the sealing surfaces on both the housing and the engine block with a clean rag. Any old gasket material or corrosion should be wiped away. Place a new gasket on the housing—most thermostats come with a gasket included, or you can buy one separately for a few dollars. Set the thermostat in place with the spring end toward the engine, then bolt the housing back down. Tighten the bolts firmly but do not overtighten, which can crack the housing.
Refill the cooling system with the same type of coolant your car uses. Check your owner's manual for the correct coolant type and the total capacity. Pour coolant into the radiator or overflow tank until it reaches the full mark. Start the engine and watch the temperature gauge to make sure it climbs to the middle and stays there. If you see steam or smell coolant, turn off the engine when ready and check for leaks around the housing bolts.
When to take your car to a mechanic
If you are not comfortable working inside the engine bay, draining coolant, or testing components, a mechanic can diagnose and replace a thermostat in under an hour. The labor cost varies by shop and car model but typically ranges from $100 to $300 on top of the cost of the part itself.
You should also see a mechanic if your temperature gauge still climbs into the red zone after you have replaced the thermostat, or if it stays low even with a new one installed. These symptoms may point to a water pump failure, a clogged radiator, or a problem with the cooling fan—issues that require different repairs.
Frequently Asked Questions
Can I drive my car if the thermostat is stuck closed?
Not safely. A stuck-closed thermostat will cause your engine to overheat within minutes, which can warp the cylinder head, damage the gaskets, and cause expensive engine damage. If you suspect a stuck thermostat, have it replaced before driving the car long distances.
What type of coolant should I use when refilling?
Check your owner's manual or the label on your radiator cap—it will specify the coolant type your car needs. Common types include green, orange, pink, and blue coolant, and they are not interchangeable. Using the wrong type can cause corrosion inside the cooling system. Most auto parts stores can tell you the correct type if you provide your car's year, make, and model.
How often should a thermostat be replaced?
A thermostat can last the life of the car if it does not fail, so there is no set replacement interval. Replace it only when it shows signs of failure or when you have tested it and confirmed it is not opening and closing correctly. Some mechanics recommend replacing it during major cooling system work, like a water pump replacement, since the housing is already open.
Is it normal for the temperature gauge to fluctuate slightly while driving?
Small fluctuations are normal as the thermostat opens and closes to maintain the engine at its target temperature. The gauge may move up and down by a few degrees. If the needle swings wildly or climbs steadily into the red, that is not normal and points to a thermostat or cooling system problem.
What happens if I install the thermostat backwards?
Installing it backwards will prevent coolant from circulating correctly, and your engine will overheat when ready. If you start the engine and the temperature gauge shoots into the red within seconds, turn it off right away. Check that the thermostat is facing the correct direction—the spring end should point toward the engine block, not toward the radiator hose.