4 Symptoms of a Bad Car Thermostat & How to Fix it?

A car cannot maintain proper engine temperature if the thermostat isn’t working right. Thermostat is a valve that cuts off or opens flow of coolant in the engine coolant chamber from the radiator.

car thermostat being pulled out for inspection
Car thermostat being pulled out of the housing

Let’s discuss symptoms of a failing or a thermostat gone kaput:

1) Overheating

Overheating engine is one of the most concerning problem due to faulty thermostat. If the thermostat remains stuck in the closed position, the coolant can’t flow to radiator to cool down.

Just driving for 15 minutes like this will bring the engine temperature needle in the red zone. And that’s it, you’ll have to stop the engine and wait for it to cool down. Keep driving an overheated car and you could cause serious damage like blown engine head gasket and warped engine cylinder. What to do if your car engine overheats.

2) Underheating

When you cold start your car, the thermostat should be in the closed state. This means that the coolant in the engine cooling channel does not flow into the radiator.

This allows the engine to reach optimal temperature quickly. If you notice that the engine is taking too long to warm up, then that could be due to thermostat stuck in open state.

3) Leaking Coolant or steaming underhood

When the engine has overheated due to thermostat in closed position, a lot of pressure gets built up as flow towards radiator is blocked.

This excess pressure can cause the thermostat housing gasket to leak. And if that coolant falls on the hot engine part, you will see steam coming from under the car hood.

Too much coolant pressure can also blow past the coolant hose. So keep an eye out for the red or green coolant pooled up under your car.

4) Fluctuating Temperature Gauge

Ideally, the temperature gauge of your car should stay in the middle green zone (between 195 degree fahrenhiet to 220 degree fahrenhiet). But, when thermostat is about to go kaput, the engine temperature might start fluctuating while driving normally.

Locating the thermostat under the hood

Thermostat housing is located between the engine and the radiator in all liquid cooled engines. Follow the upper radiator hose going towards the engine.

Thermostat is usually 2 to 3 inched in diameter. Once you’ve identified it, you need to get it out of the housing to repair to replace it.

How does a Thermostat Work?

Thermostat relies on the wax filled casing. This wax expands once the engine temperature goes over the normal range. This pushes the spring loaded valve and opens the outlet towards radiator.

And as the engine temperature starts to drop back, the wax hardens and shrinks. This allows spring loaded valve to close back.

Watch this video to understand:

Can you repair the faulty thermostat?

Yes, sometimes it’s possible to repair the faulty thermostat. Even giving it a good clean and re-installing it properly in the housing can sometimes fix the issue.

That said, thermostat is a fairly inexpensive item. And couple of bucks will get you a new one. Most mechanics don’t ever bother fiddling with the malfunctioned thermostat as it’s always better to throw in a new one.

If you do decide to work on your car yourself, keep in mind that do not work when the engine is hot.

Cost of replacing thermostat in a car

It takes around 1 hour to replace a thermostat. So, labour cost will come around $70 to $110.

The replacement thermostat itself just costs around $10 to $20

If you decide to do it yourself, it’s quite a cheap fix.

How to change a faulty thermostat?

Changing out faulty thermostat is not difficult but you need to be cautious not hurt yourself with boiling water or radiator fan.

When you replace a thermostat, it’s a good idea to put in fresh housing gaskets as well. Generally, replacing a thermostat will look like this for most vehicles.

1) Put a drain pan under the housing to catch some coolant that will drip out.
2) Loosen the hose clamps from the thermostat housing and pull it off from the housing.
3) Now, remove the bolts from the thermostat housing and take it off.
4) Thermostat can now be replaced.
5) Reassemble everything is same order
6) Since little bit of coolant dripped out, you need to check the coolant level and top it up if required.

Watch this video to understand better:

How much does it cost to replace a Thermostat?

The replacement part cost will range from $10 to $20. But the labor cost will come around $70 to $110.

Can I drive my car with faulty thermostat?

If the thermostat is stuck in closed position, the coolant will not be able to cool down. Therefore, you won’t be able to drive too far without overheating the engine. Around 2 miles is doable. If the thermostat is stuck in open position, then you can drive it normally if the weather is not too cold.

Siddharth Sharma

Siddharth has always been passionate about Cars and Bikes. He was the kind of kid that always had the latest Auto magazine in his school bag. He had this dream- to become a professional racecar driver. Finally, in 2012 he found himself racing as a rookie driver in the Polo R Cup national racing championship. Over time he had to readjust the sail and get into automotive journalism to continue enjoying machines on wheels.