How car thermostats work, and how to change one

Car thermostats play a vital role in regulating the temperature of your vehicle's engine. Without a fully functioning thermostat, the engine will be at risk of running too hot or cool, both of which can cause some serious — and potentially expensive — problems. So, if you think you might have a faulty thermostat on your hands, it's important to get it sorted as soon as possible.

In this guide, we'll explain how thermostats work in more detail, including what happens when they get stuck open or closed. We'll also talk you through the process of how to change a thermostat in case you need a full replacement.

• How car thermostats work
• What happens when a thermostat fails or gets stuck?
• Signs of a faulty thermostat
• How to change a car thermostat

How do car thermostats work?

The majority of vehicles with water-cooled engines need to maintain a consistent temperature in order to operate efficiently: this is somewhere between 75°C to 104°C for most makes and models. The function of the thermostat is to ensure the engine stays at the optimum running temperature by regulating the flow of coolant between the radiator and the engine.  

A car thermostat works by opening or closing a valve to control the flow of coolant as needed to maintain the right temperature. It uses a special kind of temperature-sensitive valve with a wax mechanism inside it. As the engine warms up, the coolant inside it gets hot, and the wax inside the valve expands and melts in the heat. This in turn pushes a small bar that splits the centre plate from the mounting base and opens the valve, allowing the coolant to circulate through the radiator, dispersing the heat. The coolant then flows back into the engine, cooling it down and stopping the car from overheating.

Once the temperature in the engine drops, the wax shrinks again, closing the valve and shutting off the flow of coolant to allow the engine to warm up again. In this way, the thermostat works to ensure that the engine never gets too hot or cool, ensuring optimum performance.

What happens when thermostat fails? Will the car run hot?

When the thermostat fails, the flow of coolant into the engine will no longer be regulated properly. So, it won't be long until the engine becomes too hot or too cold — and in some cases, it may even start to fluctuate rapidly between the two.

Whether the engine will over- or underheat depends on whether the thermostat is stuck in the open position, or whether it has jammed shut. If the thermostat is stuck in the closed position, the coolant will be trapped inside the engine and will soon get very hot, causing the engine to overheat. In the worst-case scenario, this could seriously damage or melt crucial engine components, and even cause total engine failure.

If the thermostat gets stuck in the open position, the coolant may run continuously through the engine, causing it to run too cold. An overcooled engine is much less efficient than one running at the optimum temperature, and this can have a dramatic effect on fuel efficiency. Engine parts will also wear out more quickly, and the cabin will not heat up properly, even with the heater on.

A car thermostat may fail for any number of reasons, but the most common causes tend to be problems with the water pump or radiator, or a damaged drive belt. And, if this happens, it won't be long until you start to experience a few issues with your coolant system and engine.

Signs of a faulty thermostat

A bad thermostat can be quite serious, and can even take out your entire engine, so it's important to recognise the signs that your car thermostat isn't working properly. Here, we've shared the most common symptoms of a bad thermostat.

Please note, the cooling system can be quite complex, so while the signs described below can be an indicator that the thermostat is defective, there may be other causes as well. If you're having difficulty diagnosing the issue, speak to a qualified mechanic.

Very high engine temperatures

If the engine gauge is showing extremely high running temperatures — especially within the first fifteen or twenty minutes of driving — it could be a sign of a problem with the thermostat. For vehicles that don't have an engine temperature gauge, the high engine temperature symbol may be illuminated on the dashboard. You can check what this symbol looks like for your car in the owner's manual.

Fluctuating engine temperature

If the thermostat is faulty, you may find that the engine temperature fluctuates rapidly between hot and cold. One minute, your engine temperature gauge may be in the red, then the next, it will have dropped dramatically. This is usually a sign that the thermostat has developed a fault that is preventing it from opening and shutting at the right time.

Coolant is leaking from near the thermostat

If the thermostat becomes stuck in the closed position, then the pressure can build up inside the coolant lines, and coolant may start to leak out of the system — especially in the area just around the thermostat.  You may notice a patch of coolant forming underneath your car.

If left untreated, a leak will cause the engine operating temperatures to rise. The fluid can also cause the surrounding engine parts to start to corrode or rust, which may be costly to repair if not caught early.

How to change a car thermostat in 10 steps

To replace a car thermostat, you'll need the following parts, tools, and accessories:

• A replacement thermostat: This must be the correct size, type, and brand for your vehicle. You can use the search function on our car thermostats page to find the right part for your make and model. Alternatively, look in your car owner's manual.
• A replacement thermostat gasket: You will need to replace the gasket and seals when replacing the thermostat. Most thermostats come with these as standard, but it’s also sensible to double check so you’re not caught short.
• Sealing compound. It’s sometimes necessary to use a sealing compound on the gasket. Check your manual and any guidelines that come with the part to learn more about which type to use.
• A large bucket or bowl.
• Drip tray or container. This needs to be large enough to hold the entire volume of the radiator — for most vehicles, this is around 10 litres, but it may be more for some models.
• Trolley jack.
• Axle stands.
• Ratchet and socket set.
• Assorted screwdrivers.
• Rags and cleaning cloths.
• Plastic scraper.
• Fresh coolant: You will need fresh coolant — made up of antifreeze mixed with equal parts water — to top up the system. Consult the manufacturer's manual for the exact fill volume of your cooling system.

Please note: The engine must be cold when you replace the thermostat. Never attempt this job straight after driving or when the engine is still warm, as there is a risk of scalding or burns.

The information here is intended as a general guide, and the exact process for your vehicle may vary. So, always double check in your car owner's manual or Haynes manual before you begin.

Now that you've got everything you need at the ready, let’s get started!

Step 1 – Prep the vehicle. When the car is parked on a level surface and the engine is cold, disconnect the battery. Raise the vehicle on jack stands. Open the bonnet.

Tip: Grounding the battery by removing the negative battery terminal before any job is always a sensible idea. This way, you won't accidentally complete the circuit and give yourself a shock.

Step 2 – Drain the coolant. Unscrew the radiator cap and drain out the coolant into a large container. You can learn more about how to do this in our guide to flushing a car radiator. Remember that coolant fluid is considered to be hazardous waste and must be disposed of at the appropriate recycling facility. 

Tip: If the coolant is very fresh and in good condition, you may be able to collect it in a secure container and re-use it. If not, you will be better off using fresh coolant mixture.

Step 3 – Locate the thermostat. It's normally located at the end of the cylinder head as viewed from the driver's seat. Depending on the make and model of your car, you may need to remove other housing or parts — such as the air filter assembly — in order to access the thermostat. You can refer back to your car owner's manual to help you with this.

Step 4 – Disconnect the hoses and any sensors. Remove any fittings, screws, or pins holding the upper and lower radiator hoses in place and disconnect them from the thermostat housing. If this is tricky, you may find that carefully inserting a thin, flat-head screwdriver between the hose and the fitting helps to loosen it up. Coolant may spill out when you do this, so have your bowl or basin to hand to catch any liquid.

Disconnect any other hoses and the temperature sensor, as well as any other sensors or cables linked to the thermostat. Once everything is disconnected, examine the hoses to see if they are damaged or corroded at all — if so, they will need replacing.

Step 5 – Remove the thermostat housing. Unfasten any spring clips or pins and use a socket wrench to unscrew the mounting bolts holding the thermostat housing in place. Your owner's manual can help you locate these if you are having difficulty finding them. Be sure to use the correct socket size to avoid rounding off the bolt heads. Once removed, set the bolts to one side.

Step 6 – Remove the old thermostat. Lift off the thermostat housing. Before removing the thermostat, take a moment to look at it and make a note of the way the thermostat aligns inside the housing, as the replacement must be installed in the same position.

Tip: If lifting the thermostat housing proves difficult, tap it lightly with a wooden stick or tool to loosen it — but be very gentle, as the housing is often made from aluminium or plastic and can easily be dented or damaged.

Step 7 – Clean the mating surfaces and apply new seals. The mating surfaces must be completely clean, so wipe them with a soft cloth — take care not to scratch it or damage it. It may help to use plastic scraper to remove stubborn traces of the gasket or sealing compound.

Fit new seals. If the gasket or seal requires a sealing compound, apply this now.

Step 8 – Fit the new thermostat. Compare the new and old thermostat — they should be of an identical size and shape. If they match up, install the new thermostat. When inserting the thermostat, make sure the bleed pin is pointing in the same direction as the old one.

Step 9 – Secure the housing and refit the pipes and hoses. Replace the housing and gasket or seal into position in the engine compartment. Tighten the housing nuts alternately, to avoid damaging the housing. Be careful not to overtighten them — you should be able to find torque values for them in your owner's manual.

Refit all hoses and pipes, as well as the temperature sensor, and secure. Refit any parts or housing that you removed in order to access the thermostat.

Step 10 – Finally, refill the system with fresh coolant. Remember to bleed the system to remove any air bubbles.

Hopefully this guide will have told you everything you need to know about how a car thermostat works, as well as how to change them. To learn more about maintaining your vehicle's cooling system, read our guides to spotting and repairing car radiator leaks.

Remember, here at GSF Car Parts, you'll find a selection of engine parts and cooling system parts for a huge variety of makes and models. Simply enter your registration number into the box on the homepage and find specific recommendations for your car. For more helpful advice, be sure to check out our knowledge hub. Got a question? Don't hesitate to get in touch via our contact form. Alternatively, you can call us on 0121 626 7971.