How to Replace a Thermostat

A $20 part that a shop charges $150 to $400 to fit — when it is reachable. On some engines it is buried, and then it is not worth attempting.

• Time: 1–2 hours if accessible
• Difficulty: intermediate — and entirely dependent on where yours sits
• Shop: $150–$400. Part: $15–$50. Saves: $100–$350

What a thermostat does, and how it fails

It is a temperature-operated valve between the engine and the radiator. Cold engine, it stays shut so the engine warms up quickly. Once warm, it opens and lets coolant circulate through the radiator.

• Stuck closed — coolant never reaches the radiator and the engine overheats, often quite fast. The dangerous failure.
• Stuck open — the engine never properly warms up. The temperature gauge sits low, the heater blows lukewarm, fuel economy suffers, and you may get a check engine code such as P0128.

Is yours reachable?

Follow the upper radiator hose from the radiator back toward the engine. Where it meets the engine there is usually a housing held on by two or three bolts. The thermostat is inside that housing.

• If you can see and reach those bolts from above — good. This is a straightforward job.
• If the housing is at the bottom, or behind the alternator, or under the intake — and on plenty of engines it is — the labour is the whole cost, and the shop price starts to look reasonable.
• Some newer cars have an electronically controlled thermostat integrated into a housing assembly, sometimes with a sensor plug. Those are pricier parts and sometimes need a scan tool afterwards.

Look up a video for your exact engine before buying anything. Two minutes of watching decides whether this is your afternoon or a shop’s.

What you need

• Thermostat for your exact vehicle, and its gasket or O-ring — often supplied, sometimes separate. Never reuse the old one.
• Coolant of the correct type, pre-mixed, at least a gallon
• A drain pan, sockets, a scraper or plastic razor, and rags
• A torque wrench — housings are often aluminium or plastic and crack easily

The job

1. Completely cold engine. Not warm. The system is pressurised when hot and coolant scalds badly.
2. Drain some coolant. You only need the level below the thermostat housing, so partially draining via the radiator petcock is usually enough. Catch it — coolant is toxic and is famously sweet-tasting to pets. Clean up any spills immediately.
3. Photograph the housing, then remove the hose clamp and work the hose off. Old hoses stick; twist rather than lever, and replace the hose if it is hard or mushy while you are here.
4. Undo the housing bolts and lift the housing off. Expect a little more coolant.
5. Note how the old thermostat sits — take a photo. Which way it faces matters, and there is usually a small jiggle valve or air bleed pin that has to go at the top.
6. Clean both mating faces completely. Old gasket material left behind is the number one cause of a leak afterwards. Use a plastic scraper on aluminium, not a steel blade.
7. Fit the new thermostat the same way round, with the new gasket or O-ring, jiggle valve at the top.
8. Refit the housing and torque the bolts evenly to the manual’s figure, working across rather than round. These are often only 7 to 10 lb-ft — genuinely light. Overtightening cracks the housing.
9. Reconnect the hose and clamp.

Refilling and bleeding — the part people skip

Air trapped in a cooling system causes overheating just as effectively as no coolant at all. Bleeding it out is not optional, and it is why this job goes wrong for people who otherwise did everything right.

1. Look up your car’s bleed procedure — many have a bleed screw near the thermostat housing or on a hose, and some need the front of the car raised so the filler is the highest point.
2. Fill slowly at the radiator or the expansion tank, whichever your car uses.
3. Start the engine with the cap off (if your system is designed that way) and the heater on full hot. Let it reach operating temperature and watch the level drop as the thermostat opens — keep topping up.
4. Squeeze the upper radiator hose a few times to help air work its way out.
5. Once the fan has cycled on and off and the level is stable, cap it.
6. Check the level again the next morning when cold, and watch the temperature gauge closely for the next few drives.

Dispose of the old coolant properly — many parts stores and municipal recycling centres accept it. Never down a drain.

The mistakes that catch people

• Not bleeding the air out. The car overheats and you assume the new part is faulty.
• Fitting it backwards, or with the jiggle valve at the bottom.
• Reusing the old gasket, or not cleaning the face properly.
• Overtightening and cracking the housing.
• Mixing coolant types. Use what the manual specifies.
• Replacing the thermostat when the real fault was a leak or a dead fan.

Information only, not professional advice, and no substitute for your vehicle’s service manual. Work on your own vehicle is at your own risk. See the disclaimer.