Replacing a car thermostat is a job most owners can do at home with basic tools, though the exact steps and difficulty depend on your vehicle's engine layout
A thermostat regulates coolant flow through your engine to keep it at the right temperature. When it fails, your engine either runs too hot or too cold, and your temperature gauge will show it. Replacement usually takes one to three hours and costs $15 to $50 in parts if you do it yourself, versus $150 to $400 at a shop.
The thermostat sits inside a housing where the upper radiator hose connects to the engine. You'll need to drain some coolant, unbolt the housing, swap out the old thermostat for a new one, and refill. The hardest part is usually access — on some cars the housing is right there; on others you're working around the engine bay.
Key Takeaways
- You will need to drain at least some coolant before opening the thermostat housing, so have a drain pan and a way to dispose of old coolant properly.
- The thermostat housing bolts are usually 10mm or 13mm and sit where the upper radiator hose connects to the engine block.
- Buy the correct thermostat for your year, make, and model — thermostats open at different temperatures and are not interchangeable.
- After installation, refill the coolant system and run the engine to bleed air from the lines, or you'll have overheating problems.
- If you cannot reach the housing without removing other parts, or if your vehicle has an integrated thermostat in the water pump, this job is better left to a shop.
What you'll need before you start
Gather your tools and materials before you begin. You will need a socket set (usually 10mm and 13mm sockets), a wrench or ratchet, a drain pan large enough for several quarts of coolant, a new thermostat matched to your vehicle, a gasket or O-ring (often included with the thermostat), and a coolant funnel. Some cars also need a gasket scraper or plastic pry tool to remove the old gasket without damaging the housing.
Check your owner's manual or a repair guide specific to your vehicle to find the thermostat location and see if any other components need to come off first. Some engines have the housing in plain sight; others hide it behind the alternator, power steering pump, or intake manifold. If removal requires more than unbolting the housing and disconnecting the hose, you may want to stop and take it to a shop.
Let your engine cool completely before you start. A hot engine and pressurized coolant system can cause serious burns. If the engine is warm, wait at least an hour.
Draining the coolant safely
Locate the radiator drain plug at the bottom of the radiator or the engine block. Place your drain pan underneath and open the plug by turning it counterclockwise. Let the coolant drain until it stops flowing — you don't need to drain the entire system, just enough to drop the coolant level below the thermostat housing.
Old coolant is toxic and should never go down a storm drain or into the ground. Many auto parts stores and service stations accept used coolant for free or a small fee. Some municipalities have hazardous waste collection days. Call ahead to confirm where you can dispose of it before you drain it.
Once the coolant stops dripping, close the drain plug and hand-tighten it. Do not over-tighten or you may crack the radiator.
Removing the thermostat housing and old thermostat
Disconnect the upper radiator hose from the thermostat housing by loosening the hose clamp with a screwdriver or wrench. You may need to twist the hose gently to break it free. Have a rag ready — coolant will still drip from the hose.
Unbolt the thermostat housing using your socket set. Most housings are held by two or three bolts. Keep the bolts in a safe place so you do not lose them. Carefully lift the housing away from the engine block.
Inside the housing you will see the old thermostat — it looks like a brass or metal cylinder with a spring. Remove it by hand or with a gentle pry if it is stuck. Scrape away the old gasket material from both the housing and the engine block using a gasket scraper or plastic pry tool. Do not use a metal blade that could gouge the sealing surface. Wipe the surfaces clean with a rag.
Installing the new thermostat and housing
Check that your new thermostat matches the temperature rating stamped on the old one — common ratings are 180°F, 192°F, and 203°F. If the ratings do not match, you have the wrong part and should return it.
Place the new gasket or O-ring on the thermostat housing. Some thermostats come with a gasket already attached; others require you to install one separately. If your kit includes a small tube of gasket maker, explore a thin bead around the sealing surface of the housing before you install the gasket.
Insert the new thermostat into the engine block with the spring facing into the block. Set the housing back in place and install the bolts. Tighten them in a crisscross pattern — tighten one bolt partway, then the opposite bolt, then back to the first — to may support even pressure. Tighten firmly but do not over-tighten, which can crack the housing. Most housing bolts need only 15 to 25 foot-pounds of torque.
Reconnecting the radiator hose and refilling coolant
Slide the upper radiator hose back onto the thermostat housing and tighten the hose clamp. The clamp should be snug enough that the hose does not slide, but not so tight that you crush the hose.
Close the radiator drain plug if you opened it. Place a coolant funnel in the radiator filler neck and pour in a 50/50 mix of coolant and distilled water until the radiator is full. Check your owner's manual for the correct coolant type — using the wrong type can damage your engine. Pour slowly to avoid air pockets.
Start the engine and let it idle for several minutes. You will hear the cooling fan kick on as the engine warms up. Watch the temperature gauge — it should climb slowly and stay in the normal range. If the gauge spikes into the red zone, stop the engine when ready and let it cool; you likely have an air pocket in the system.
Once the engine reaches normal operating temperature, turn it off and let it cool for at least an hour. Check the coolant level again and top off if needed. Air pockets often rise to the surface after the first heat cycle, lowering the level.
Checking your work and troubleshooting common problems
After the engine has cooled, inspect the thermostat housing for leaks around the bolts or gasket. A small weep is normal for the first few days as the gasket settles, but steady dripping means the housing bolts need tightening or the gasket was not seated properly.
If your temperature gauge still reads too hot or too cold after replacement, the new thermostat may be faulty or you may have installed it backwards. The thermostat should go in with the spring facing the engine block. If the gauge fluctuates wildly or the engine overheats, you likely have air trapped in the cooling system. Some vehicles need to be bled by opening a small vent screw on the thermostat housing or heater hose while the engine runs — check your repair manual for the location.
If you see white or green coolant pooling under the car, you have a leak. Check that the radiator hose clamp is tight and that the housing bolts are snug. If the leak persists after tightening, the gasket may not be seating correctly and the housing may need to come off again.
When to take it to a shop instead
Some vehicles have thermostats integrated into the water pump or buried so deep in the engine bay that removing them requires taking off the alternator, power steering pump, or intake manifold. If your repair manual shows that more than three bolts need to come out before you can reach the housing, or if the housing is not visible from above the engine, this job is better handled by a technician with a lift and specialized tools.
If you are uncomfortable working with coolant, do not have a safe place to dispose of old coolant, or lack the basic socket set and wrenches, a shop visit will cost more but will be faster and safer. A thermostat replacement at a shop typically takes one to two hours of labor.
Frequently Asked Questions
Can I drive the car right after replacing the thermostat?
Yes, but only after the engine has warmed up and you have checked the coolant level. Drive it gently for the first 10 to 15 minutes to make sure the temperature gauge stays in the normal range. If the gauge climbs into the red zone, pull over, turn off the engine, and let it cool before driving to a shop.
What if coolant leaks from the housing after I bolt it back together?
Stop the engine and let it cool completely. Tighten the housing bolts in a crisscross pattern — do not tighten one bolt all the way and then move to the next. If tightening does not stop the leak, the gasket may not be seated correctly and the housing will need to come off again to reseat it or replace the gasket.
How do I know if I bought the right thermostat?
Check the temperature rating stamped on the old thermostat before you remove it, and buy a new one with the same rating. Common ratings are 180°F, 192°F, and 203°F. Using the wrong temperature can cause overheating or poor fuel economy. Your parts store can also look up the correct thermostat by your vehicle's year, make, and model.
Do I need to bleed air from the cooling system?
Most cars bleed air naturally as the engine warms up. However, some vehicles have a small vent screw on the thermostat housing or heater hose that needs to be opened while the engine idles to release trapped air. Check your repair manual or ask your parts store whether your vehicle needs manual bleeding.