How To Check The Thermostat On A Car

If your engine seems to run too hot or too cold, testing the thermostat is a logical first step. This guide will show you exactly how to check the thermostat on a car, a simple diagnostic task that can save you time and money. A faulty thermostat is a common culprit behind temperature problems, and verifying its operation is often straightforward.

You don’t need to be a master mechanic to perform this check. With some basic tools and safety precautions, you can determine if your thermostat is stuck open, stuck closed, or working correctly. Let’s get started.

There are two primary methods to check your car’s thermostat: one with the engine running and one with it off. We’ll cover both. The running test is a good initial check, while the removal test provides definitive proof. Always ensure the engine is completely cool before you begin any work to avoid burns or injury.

Understanding Your Car’s Thermostat

Before you start testing, it helps to know what you’re dealing with. The thermostat is a temperature-controlled valve located between your engine and the radiator. Its job is to regulate coolant flow.

When the engine is cold, the thermostat stays closed. This allows the engine to warm up quickly by blocking coolant from circulating to the radiator. Once the engine reaches its optimal operating temperature—typically between 195°F and 220°F (90°C and 105°C)—the thermostat opens. This lets coolant flow through the radiator to be cooled before returning to the engine.

Common Symptoms of a Bad Thermostat

Engine Overheating: A thermostat stuck in the closed position will prevent coolant from reaching the radiator, causing rapid overheating.
Engine Runs Too Cool: A thermostat stuck open means coolant constantly flows, preventing the engine from reaching its proper operating temperature. You might notice poor heater performance or reduced fuel efficiency.
Erratic Temperature Gauge: The temperature needle may fluctuate wildly between hot and cold.
Coolant Leaks: A failing thermostat housing gasket can lead to visible coolant leaks around the thermostat housing.

Method 1: The Warm-Up Test (Engine Running)

This is the simplest way to check thermostat function without removing any parts. It requires you to safely observe the engine as it warms.

Ensure a Cold Start: Make sure your engine has been off for several hours and is completely cool to the touch. Check that the coolant level in the overflow reservoir is correct.
Locate the Upper Radiator Hose: Open the hood and find the large upper hose that connects the top of the radiator to the engine. This hose carries hot coolant from the engine to the radiator.
Start the Engine: With the hood open and the parking brake set, start the engine. Let it idle. Do not rev the engine.
Feel the Radiator Hose: Carefully feel the upper radiator hose as the engine runs. For the first few minutes, the hose should remain cool and soft. This indicates the thermostat is closed, blocking coolant flow.
Watch for the Change: After 5 to 15 minutes, as the temperature gauge on your dashboard begins to rise, the upper radiator hose should suddenly become very hot and firm to the squeeze. This is the moment the thermostat opens, allowing hot coolant to surge into the hose.

If the hose gets hot immediately when the engine starts, the thermostat is likely stuck open. If the hose never gets hot and the engine begins to overheat, the thermostat is probably stuck closed. If you feel a distinct change from cool to hot at roughly the normal operating temperature, the thermostat is working.

Method 2: The Removal And Boiling Water Test

For a conclusive test, you can remove the thermostat and inspect it physically. This method confirms its opening temperature and mechanical function.

Tools and Materials You Will Need

Basic socket set and wrenches
A drain pan
New thermostat gasket (and possibly new thermostat)
Fresh coolant (to top up the system)
A pot, water, a stove, and a cooking thermometer
Pliers or a small pick

Drain Some Coolant: Place the drain pan under the radiator or thermostat housing. Carefully open the radiator drain petcock or loosen the lower radiator hose clamp to drain about a quart or two of coolant. This prevents a large spill when you open the thermostat housing. Always dispose of old coolant properly.
Locate and Remove the Thermostat Housing: The thermostat housing is usually where the upper radiator hose connects to the engine. It may be made of metal or plastic. Use your socket set to remove the bolts holding the housing on. Gently pry the housing loose.
Remove the Thermostat: Inside, you’ll find the thermostat. Note its orientation—which way is up. Use pliers or your fingers to lift it out. Clean the mating surfaces on the housing and engine block of any old gasket material.
Visual Inspection: Look at the thermostat. Check for obvious corrosion, mineral deposits, or a stuck valve. The main valve (a metal disc) should be closed.
The Boiling Water Test: Fill your pot with water and place the cooking thermometer inside. Suspend the thermostat in the water using a string or wire, ensuring it doesn’t touch the bottom or sides of the pot. Do not rest it on the bottom, as this can affect the reading.
Heat the Water: Slowly heat the pot on the stove while watching the thermometer. The thermostat should begin to open just as the water reaches the temperature stamped on its flange (e.g., 195°F). You should see the valve disc move noticeably.
Check Full Opening: Continue heating the water about 20°F above its rated temperature. The valve should be fully open. Remove the thermostat from the water with tongs and observe it. As it cools, the valve should slowly close completely.

If the thermostat fails to open at its rated temperature, opens too early, or does not close upon cooling, it is faulty and needs replacement. If it passes the test, the issue may lie elsewhere in the cooling system.

Reinstalling Or Replacing The Thermostat

If your thermostat passed the test, you can reinstall it. If it failed, install a new one. Always use a new gasket to prevent leaks.

Place the new or tested thermostat into the engine block in the correct orientation (usually with the spring side facing into the engine).
Apply a thin bead of gasket maker to the new gasket if recommended, or simply place the clean gasket over the housing.
Carefully position the housing back onto the engine and hand-tighten the bolts.
Tighten the bolts in a criss-cross pattern to the manufacturer’s specification to avoid warping the housing. Overtightening, especially on plastic housings, is a common mistake.
Refill the cooling system with the drained coolant and any fresh coolant needed. Follow your vehicle’s specific bleeding procedure to remove air pockets, as air in the system can cause overheating.
Start the engine with the radiator cap off (if possible) and let it reach operating temperature. Watch for leaks and check that the heater blows hot air, confirming good coolant circulation.

Important Safety Tips And Common Mistakes

Working on the cooling system requires caution. Here are key points to remember.

Never Open a Hot System: The cooling system is under high pressure when hot. Opening the radiator cap or thermostat housing on a hot engine can cause severe burns from erupting steam and coolant.
Use Proper Coolant: Always use the type of coolant specified in your owner’s manual. Mixing different types can cause gel formation and clog the system.
Bleed the System: After any cooling system service, air must be bled out. Consult your repair manual for the specific procedure for your car, as methods vary. An unbled system will overheat.
Check Related Components: While you’re there, inspect the radiator hoses for cracks or soft spots and check the condition of the drive belts. A weak water pump can also cause cooling issues.

When To Call A Professional Mechanic

While checking a thermostat is generally a DIY-friendly job, certain situations warrant professional help. If you are uncomfortable working on your engine, if the thermostat housing is in an extremely difficult location, or if you’ve replaced the thermostat and the problem persists, it’s time to seek an expert. They can perform a pressure test on the system and diagnose more complex issues like a failing water pump, clogged radiator, or a blown head gasket.

Frequently Asked Questions (FAQ)

How Do I Know If My Car Thermostat Is Bad?

The main signs are the engine running hotter than normal or not reaching normal temperature, a fluctuating temperature gauge, and poor heater output. The warm-up test with the radiator hose is a good way to confirm your suspicions.

Can You Test a Thermostat Without Removing It?

Yes, you can perform the initial warm-up test by feeling the upper radiator hose as described. This gives a strong indication of function but is not as definitive as the removal and boiling water test.

What Happens If You Drive With a Bad Thermostat?

Driving with a thermostat stuck closed will lead to severe overheating, which can cause catastrophic engine damage like a warped cylinder head or blown head gasket. Driving with one stuck open reduces efficiency and increases engine wear over time due to constant operation at low temperatures.

How Much Does It Cost to Replace a Car Thermostat?

The part itself is usually inexpensive, often between $15 and $50. If you have a professional do the job, labor typically adds another $100 to $250, depending on the vehicle’s make and model and the labor rates in your area.

How Long Does a Car Thermostat Usually Last?

Thermostats don’t have a fixed service interval, but they can fail at any time. Many last 10 years or 100,000 miles, but corrosion, contaminated coolant, or simple wear can cause premature failure. It’s a relatively common repair item.

Learning how to check your car’s thermostat is a valuable skill. It empowers you to diagnose a common problem, avoid unnecessary repairs, and keep your engine running at its ideal temperature. Regular maintenance of your cooling system, including periodic coolant changes, is the best way to prevent thermostat and other cooling-related issues down the road.