How To Check Thermostat In Car

Knowing how to check thermostat in car is a crucial skill for any driver. A faulty thermostat can cause your engine to overheat or never reach its proper operating temperature. This simple, inexpensive part controls the flow of coolant and is vital for engine health. Learning to test it yourself can save you time, money, and prevent a breakdown.

This guide will walk you through several reliable methods. You will learn to recognize the symptoms of a bad thermostat and perform checks safely. We cover everything from visual inspections to temperature tests you can do at home.

Checking your car’s thermostat involves a few straightforward procedures. You do not always need to remove it from the vehicle to determine if it’s working. The following methods range from simple observation to hands-on testing. Always ensure the engine is completely cool before starting any work to avoid burns.

Recognizing The Symptoms Of A Faulty Thermostat

Before you start testing, it helps to know the warning signs. A failing thermostat typically shows clear symptoms related to engine temperature. Pay attention to these common indicators.

Engine Overheating

This is the most obvious sign. If the thermostat sticks closed, coolant cannot circulate to the radiator. The engine temperature gauge will climb into the red zone, and you may see warning lights.

Engine Never Reaches Normal Temperature

Conversely, a thermostat stuck open allows constant coolant flow. The engine will take a very long time to warm up, or it may never reach its optimal operating temperature on the gauge.

Erratic Temperature Gauge Behavior

The temperature needle may fluctuate wildly, jumping from cold to hot and back again. This often indicates a thermostat that is intermittently sticking.

Poor Heater Performance

Since your car’s heater relies on hot coolant, a stuck-open thermostat means the coolant never gets hot enough. This results in weak or cold air from the heater vents.

Coolant Leaks Around The Thermostat Housing

Visible coolant seepage or crusty deposits around the thermostat housing can signal a failing gasket or a housing cracked from repeated overheating.

Safety Precautions And Tools You Will Need

Working with a hot engine and coolant requires caution. Gather these tools and follow these safety steps to ensure a smooth, safe process.

Allow the engine to cool completely. Never open the radiator cap on a hot engine.
Wear safety glasses and gloves to protect from coolant splash.
Have a drain pan ready to catch any spilled coolant.
Basic hand tools: sockets, wrenches, and a screwdriver set for your vehicle.
A new thermostat and gasket (for the replacement test).
A infrared thermometer or a reliable cooking thermometer for temperature checks.
Fresh coolant to top up the system after testing.

Method 1: The Visual And Touch Inspection (Cold Engine)

This is a quick, initial check you can perform. It involves locating the thermostat and feeling for temperature differences in the cooling system hoses.

Open the hood and locate the thermostat housing. It is usually where the upper radiator hose connects to the engine.
With the engine cold, squeeze the upper radiator hose. It should be firm and not contain any coolant if the thermostat is closed.
Start the engine and let it idle. Watch the temperature gauge closely.
After a few minutes, carefully feel the upper radiator hose again. It should remain cool for several minutes as the engine warms.
Once the engine’s temperature gauge begins to move toward normal, the upper hose should quickly become hot. This indicates the thermostat has opened and hot coolant is now flowing.

If the upper hose gets hot immediately upon starting, the thermostat is likely stuck open. If the engine temperature rises but the hose stays cool for a long time, the thermostat is probably stuck closed.

Method 2: The Boiling Water Test (Removed Thermostat)

This is the definitive test for a thermostat’s mechanical function. You will need to remove the thermostat from the car for this procedure.

Drain a small amount of coolant from the radiator to below the level of the thermostat housing.
Disconnect the upper radiator hose from the thermostat housing.
Remove the bolts securing the thermostat housing and carefully lift it off.
Take out the thermostat, noting its orientation. Clean the mounting surface.
Suspend the thermostat in a pot of room-temperature water using a string or a wire hook. Do not let it touch the bottom or sides of the pot.
Place a cooking thermometer in the water so you can monitor the temperature.
Slowly heat the water on a stove, stirring gently.
Observe the thermostat valve. It should begin to open at the temperature stamped on its housing (usually between 180°F and 195°F).
Continue heating. The valve should open fully about 15-20 degrees above its rated opening temperature.
Remove the pot from heat and watch the thermostat. As the water cools, the valve should slowly close completely.

A thermostat that fails to open, opens at the wrong temperature, or does not close is faulty and must be replaced. This test gives you a clear visual confirmation of its operation.

Method 3: Using An Infrared Thermometer

An infrared (IR) thermometer allows for a precise, non-contact test without removing the thermostat. This is a highly effective method.

Start with a cold engine. Point the IR thermometer at the thermostat housing and note the temperature.
Start the engine and let it idle. Periodically check the temperature at the thermostat housing and at the upper radiator hose near the housing.
As the engine warms, the housing temperature will rise steadily.
When the housing temperature reaches the thermostat’s rated opening temperature (e.g., 190°F), immediately check the upper radiator hose temperature right next to the housing.
You should observe a sharp increase in the hose temperature, indicating hot coolant has started to flow through it.

If the hose temperature remains cool well past the thermostat’s rated opening point, the thermostat is stuck closed. If the hose temperature rises gradually from the moment you start the engine, the thermostat is stuck open.

What To Do If Your Thermostat Is Faulty

If your tests confirm a bad thermostat, replacement is the only option. They are not repairable. Follow these general steps for a successful replacement.

Purchase the correct thermostat and gasket for your specific vehicle make, model, and engine.
Completely drain the cooling system from the radiator drain plug or lower hose.
Remove the old thermostat and thoroughly clean the mating surfaces on the engine and housing.
Install the new thermostat in the correct orientation (the spring side goes into the engine block).
Place the new gasket and reinstall the housing, tightening the bolts evenly to the manufacturer’s specification to avoid leaks.
Refill the cooling system with the correct type and mixture of coolant.
Start the engine with the radiator cap off to allow air to escape. Top up the coolant as needed.
Run the engine to operating temperature with the heater on high to purge any remaining air from the system.
Check for leaks and verify the temperature gauge operates normally.

Common Mistakes To Avoid When Checking Your Thermostat

Even simple checks can go wrong if you’re not careful. Avoid these common pitfalls to get accurate results and stay safe.

Opening the radiator or coolant reservoir cap on a hot engine. This can cause severe burns from erupting steam and coolant.
Assuming overheating is always the thermostat. It could also be a bad water pump, clogged radiator, or low coolant.
Forgetting to burp the cooling system after replacement. Trapped air can cause overheating.
Overtightening the thermostat housing bolts, which can crack the housing.
Installing the thermostat backwards, which will prevent it from opening correctly.
Using the wrong type of coolant or mixing incompatible types.

When To Seek Professional Help

While checking a thermostat is generally a DIY-friendly task, some situations warrant a trip to a mechanic. Do not hesitate to seek help if you encounter the following.

You are uncomfortable working on your car’s cooling system.
The thermostat housing is severely corroded or broken.
You have followed the tests but the engine still overheats, indicating a deeper problem.
There is significant coolant contamination, such as oil in the coolant, suggesting a more serious internal engine issue.
You lack the necessary tools to perform the job safely and correctly.

Frequently Asked Questions About Car Thermostats

How Much Does It Cost To Replace A Car Thermostat?

The cost varies widely. The part itself is inexpensive, usually between $15 and $50. If you have a mechanic do the job, labor can add $100 to $300, depending on your vehicle’s make and model and the shop’s hourly rate. Doing it yourself saves the labor cost.

Can You Drive With A Bad Thermostat?

It is strongly discouraged. Driving with a thermostat stuck closed will lead to rapid overheating and can cause catastrophic engine damage like a warped cylinder head or blown head gasket. Driving with one stuck open reduces fuel efficiency, increases engine wear, and can damage the catalytic converter over time due to unburned fuel.

How Often Should A Car Thermostat Be Replaced?

Thermostats are not a routine maintenance item. They are typically replaced only when they fail. However, they can last anywhere from 5 to 10 years. Many mechanics recommend replacing the thermostat when doing a major cooling system service, like a timing belt change or radiator replacement, as a preventative measure since the part is low-cost and the system is already drained.

What Are The Signs Of A Thermostat Stuck Open?

The primary signs include the engine taking an unusually long time to warm up, the temperature gauge reading low or fluctuating, and poor heater performance. You may also notice decreased fuel economy because the engine is constantly running in a “warm-up” mode, which uses more fuel.

Is It Hard To Change A Thermostat In A Car?

For most front-wheel-drive vehicles with the engine mounted transversely, it is a moderately easy job. The thermostat is often accessible near the top of the engine. In some rear-wheel-drive or luxury cars, the thermostat may be buried under other components, making the job more difficult and time-consuming. Always consult a service manual for your specific vehicle before beginning.