What a car thermostat does and why it fails

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 it reaches the right temperature—usually around 195 to 220 degrees Fahrenheit depending on your car—the thermostat opens to let coolant circulate and prevent overheating.

A thermostat fails when the valve gets stuck either open or closed. If it sticks open, your engine runs cold and your heater blows lukewarm air. If it sticks closed, coolant cannot flow, your engine overheats, and you risk serious damage. Most thermostats last between 80,000 and 100,000 miles, but some fail much earlier.

Replacing one yourself takes two to four hours and costs $40 to $150 for the part, depending on your vehicle. A shop will charge $200 to $500 total because of labor. The job is straightforward on many cars but requires draining coolant, so plan to dispose of the old coolant properly—most auto parts stores take it free.

Key Takeaways

  • The thermostat housing is usually bolted to the engine block near where the upper radiator hose connects, and you must drain the coolant system before opening it.
  • You will need a socket set, a drain pan, a gasket scraper, and a new gasket or sealant because the old gasket cannot be reused.
  • The thermostat itself is a small cylindrical part that sits inside the housing; note which direction it faces before you remove it so you install the new one the same way.
  • After installation, refill the coolant, start the engine, and check that the upper radiator hose gets hot within a few minutes—if it stays cold, the new thermostat may be stuck closed.

Locate the thermostat housing on your engine

The thermostat lives inside a housing bolted to the engine block. On most cars, this housing is at the end of the upper radiator hose where it connects to the engine. Open your hood and trace the thick rubber hose from the top of the radiator toward the engine; the housing is usually where that hose attaches.

Some vehicles have the housing on the side of the block or even near the water pump. Check your owner's manual or a repair guide for your specific year, make, and model to confirm the exact location. Taking a photo of the hose routing before you start helps you remember how to reconnect everything.

Drain the coolant and remove the housing

Park on level ground and let the engine cool completely—hot coolant will burn you. Locate the drain plug at the bottom of the radiator and place a drain pan underneath. Open the plug and let all the coolant drain into the pan. Some vehicles also have a drain plug on the engine block; check your manual.

Once drained, unbolt the thermostat housing from the engine block. Most housings are held by two or three bolts. As you remove the last bolt, support the housing with your other hand so it does not fall. Disconnect the upper radiator hose from the housing if it is in the way, but you do not have to remove the entire hose.

Lift the housing away and set it on a clean surface. Scrape off the old gasket with a gasket scraper or plastic putty knife—do not use a metal blade on aluminum surfaces, as it will gouge them. Wipe the surface clean with a rag and let it dry completely.

Remove the old thermostat and install the new one

Inside the housing, you will see the thermostat—a small cylindrical part, usually brass or stainless steel, about the size of a large coin. Note which direction it faces: most have a small air bleed hole or a spring that points a specific way. Take a photo or make a sketch so you install the new one identically.

Lift the old thermostat straight out. It may be tight; wiggle it gently if needed. Clean the inside of the housing with a rag to remove any debris or old gasket material. Place the new thermostat into the housing in the exact same orientation as the old one. If the new thermostat came with a rubber seal, make sure it is seated properly in the groove.

Some thermostats are held in place by a small clip or spring; check your manual to see if yours needs one. If your new thermostat did not come with a gasket, you will need to buy one separately—do not reuse the old gasket.

Seal and bolt the housing back on

explore a thin bead of gasket maker or high-temperature sealant around the sealing surface of the housing, following the product instructions for drying time. If you have a new gasket, lay it on the surface instead and skip the sealant. Position the housing back against the engine block and insert the bolts by hand first to make sure they thread straight.

Tighten the bolts in a crisscross pattern—tighten one bolt a little, then the opposite bolt a little, then repeat—so the housing seats evenly. Do not over-tighten; most housing bolts need only 15 to 25 foot-pounds of torque. If you do not have a torque wrench, tighten until snug and then stop.

Reconnect the upper radiator hose to the housing if you disconnected it, and tighten the hose clamp firmly but not so tight that you crush the hose.

Refill the coolant and test the thermostat

Close the radiator drain plug and any block drain plugs. Pour the correct coolant type for your car into the radiator until it reaches the fill line. Check your owner's manual for the type—mixing types can cause problems. Replace the radiator cap.

Start the engine and let it idle. Within two to three minutes, feel the upper radiator hose—it should become hot as the thermostat opens and coolant begins to flow. If the hose stays cold after five minutes, the new thermostat may be stuck closed and will need to be replaced again.

Watch the temperature gauge on your dashboard. It should climb to the normal operating range and stay there. If the engine overheats or the temperature swings wildly, turn off the engine when ready and check your work. Once the engine reaches full temperature, turn it off and let it cool, then check the coolant level again and top off if needed.

Dispose of old coolant properly

Never pour old coolant down the drain or onto the ground—it is toxic to animals and the environment. Most auto parts stores accept used coolant for free, even if you did not buy anything from them. Call ahead to confirm they take it. Some recycling centers also accept it.

Store the drained coolant in a sealed container away from children and pets until you can take it in. Label the container clearly so no one mistakes it for something else.

Frequently Asked Questions

How do I know if my thermostat is actually broken?

If your engine runs cold and your heater blows lukewarm air even after driving for ten minutes, the thermostat is likely stuck open. If your engine overheats quickly or the temperature gauge climbs into the red zone, it is likely stuck closed. A stuck-open thermostat will not damage the engine but wastes fuel and makes your cabin cold. A stuck-closed thermostat can cause serious engine damage within minutes.

Can I drive with a broken thermostat?

If it is stuck open, you can drive carefully to a shop, though your heater will not work and fuel economy will suffer. If it is stuck closed, do not drive at all—pull over when ready when you see the temperature gauge climbing, turn off the engine, and call for a tow. Continuing to drive will overheat the engine and crack the block or warp the head.

What if I cannot get the old thermostat out?

Soak the housing with penetrating oil like PB Blaster and wait 15 to 30 minutes. Try again gently with a slight rocking motion. If it is still stuck, you may need to remove the entire housing and soak it in a parts washer or have a shop handle it. Do not force it or you risk breaking the housing.

Do I need to bleed air from the cooling system after refilling?

Most modern cars bleed air automatically as you run the engine. However, some vehicles have a bleed screw on the thermostat housing or heater hose. Check your manual; if yours has one, open it slightly while refilling coolant until coolant flows out, then close it. This prevents air pockets that can cause overheating.

What if the new thermostat still does not work?

Make sure you installed it in the correct direction—the air bleed hole or spring must face the way it did on the old one. Check that the gasket is sealing properly and the bolts are tight. If the engine still runs cold or hot, the new thermostat may be defective. Return it to the parts store and get a replacement.