How Does A Car Thermostat Work? (& How to Test it)

Ever wondered what a car thermostat does, or how you can test your own at home? Find out here!

How Does A Car Thermostat Work

The car thermostat is a small component, but it has a valuable job. It’s a crucial piece to the cooling system and is responsible for protecting the engine from unnecessary wear. The thermostat sits between the radiator and engine, connected with the lower or upper radiator hose. But, how does a car thermostat work?

In this guide, we look closer at the function of the car thermostat. We also examine the signs that something is wrong with it and show you the steps to test it. These simple tips could save you from ruining your car engine. 

How Does a Car Thermostat Work?

Most car engines from the past few years operate at the same temperature range to ensure maximum performance. In general, the car engine should be around 195 to 220 degrees Fahrenheit. When it doesn’t run within these temperature guidelines, it’s not operating efficiently. 

It’s the job of the car thermostat to keep the engine’s temperature within it’s working limits. This small part sits between the engine and radiator, regulating coolant flow between the two parts. When the thermostat closes, the coolant isn’t able to flow. When it’s open, coolant flows to dissipate heat. 

When coolant is kept from flowing to the radiator, the engine temperature increases rapidly. On the other hand, as the coolant flows into the radiator, heat is quickly dissipated. 

The car thermostat operates as a gatekeeper. It’s either allowing or blocking coolant from flowing from the motor into the vehicle’s radiator. It’s the job of the thermostat to read the temperature of the engine and determine if the coolant needs to flow or not. If the engine is hot, the thermostat opens up and allows the coolant to move. Otherwise, when the engine remains cold, the thermostat stays closed. 

Signs of a Bad Thermostat

1. Overheating Engine

Overheating Car Engine

When the car thermostat remains stuck closed, the hot coolant cannot flow to the radiator. Because the hot fluid cannot be cooled down, it keeps the temperature of the engine up higher than normal.

Coolant needs to make its way to the radiator to be cooled down. Once this occurs, it comes back to the engine and dissipates more of the heat. However, with the stuck thermostat, the coolant remains stagnant, continuing to build up more heat.

If the engine overheats, it can cause serious damage. For example, if you end up with a blown head gasket, you are looking at massive repair bills.

However, an overheating engine isn’t always caused by a bad thermostat. You want to check the coolant levels to ensure you have enough fluid in the system. It could also be due to a bad water pump or a cooling system leak. Additionally, the radiator could become clogged. Still, the thermostat should be one of the parts you examine. 

2. Trouble Reaching Operating Temperature

Temperature Gauge Stays On Cold

If the opposite were true from the above complaint, the thermostat would be stuck open. When this happens, the coolant is able to flow freely to the radiator. Without coolant sitting in the engine for an extended period of time, the operating temperature might fail to reach normal limits. Not only will the engine struggle to increase temperature, but it will also have trouble maintaining its range.

When the engine doesn’t get warm enough, it’s going to be less efficient. You might notice that the motor is using more fuel than usual, costing you additional money at the pump. This problem can also lead to premature wear of the engine components. 

3. Engine Temperature Changes

When the thermostat starts to fail, it can create an erratic operation. Sometimes it could be stuck open when it should be closed and vice versa. This malfunction leads to temperature fluctuations in the engine that aren’t normal. 

When the thermostat doesn’t fail uniformly, it can create mixed signals. You will notice strange temperature readings, leading you to wonder what is causing the problem. However, a quick check of the thermostat might reveal that this is the culprit. 

4. Poor Performance

The thermostat is a critical component designed to ensure that the engine is always running at its optimal temperature. Without it, the engine won’t operate as it should.

When the motor isn’t running right, you are going to notice performance issues. Most notably, the fuel economy is going to suffer as the engine works harder to compensate for the temperature discrepancies. 

If the engine is overheating or unable to reach temperature, there could also be increased emissions from the fault. Not only is the bad thermostat putting the engine at risk for failure, but it’s also contributing to a polluted environment. 

RELATED: 5 Symptoms of a Bad Car Thermostat

How to Test a Bad Thermostat

It doesn’t take a lot of mechanical knowledge to test the car thermostat. In fact, a simple visual exam is all you need to perform.

Start by taking off the radiator cap. It’s under the hood, near the front of the vehicle, mounted to the radiator. Don’t ever open the radiator cap when the engine is hot. This test should only be done with a cold engine. Turn the car on and allow the motor to run. 

As you look inside the neck, you should not see coolant flowing to the radiator. As the engine is cool, the fluid should not be moving inside. If you already see coolant running, the thermostat is open when it shouldn’t be. 

If the coolant isn’t flowing while the engine is cold, everything is running as it should so far. Now, you want to allow the motor the chance to warm up. However, you need to be careful with the hot coolant, as it can cause injury.

As the temperature rises in the motor, coolant will start flowing into the radiator. If you don’t see any coolant moving but the temperature is rising on the dashboard, the thermostat might be stuck closed. In either situation, you must replace the thermostat to repair the issue. 

You can also remove the thermostat and boil some water to see if it’s moving like demonstrated here in this video:

Thermostat Replacement Cost

The cost to replace a bad thermostat should be between $125 and $250. The parts usually only cost around $40 to $65, while the labor can total $85 to $195. 

The repair bill varies based on what type of vehicle you drive and how difficult it is to reach the thermostat. Additionally, the region you live in can have a dramatic impact on your auto repair costs.

If you have some basic tools and some mechanical knowledge, you could change the thermostat yourself and save some money. It shouldn’t take more than two hours in most situations.