How to Test Your Vehicle's Thermostat for Optimal Performance

fryejs91
Dec 15
3 min read

A vehicle’s thermostat plays a crucial role in regulating engine temperature. When it works correctly, it helps the engine reach and maintain the ideal operating temperature, ensuring efficiency and preventing damage. A faulty thermostat can cause overheating or poor engine performance, leading to costly repairs. Knowing how to test your vehicle’s thermostat can save you time and money by catching problems early.

Close-up view of a car thermostat removed from engine — Car thermostat removed from engine, ready for inspection

What Does a Thermostat Do?

The thermostat controls the flow of coolant between the engine and the radiator. When the engine is cold, the thermostat stays closed to allow the engine to warm up quickly. Once the engine reaches a certain temperature, usually around 195°F (90°C), the thermostat opens to let coolant flow to the radiator, preventing overheating.

If the thermostat sticks closed, the engine overheats because coolant cannot circulate. If it sticks open, the engine runs too cool, reducing fuel efficiency and increasing emissions.

Signs Your Thermostat May Be Failing

Before testing, watch for these common symptoms:

Engine temperature gauge rises above normal or fluctuates erratically
Heater inside the car blows cold air even when the engine is warm
Coolant leaks near the thermostat housing
Poor fuel economy or engine performance issues
Engine warning light comes on

If you notice any of these signs, testing the thermostat is a good next step.

How to Test Your Vehicle’s Thermostat

Testing a thermostat involves checking if it opens and closes at the right temperature. Here are two simple methods you can try at home with basic tools.

Method 1: Testing Thermostat in Boiling Water

What you need:

A pot of water
A thermometer
Pliers or tongs
A stove or heat source

Steps:

Remove the thermostat from the vehicle. This usually involves draining some coolant and loosening bolts on the thermostat housing. Consult your vehicle’s manual for exact instructions.
Attach a thermometer to the pot of water and heat it slowly.
Use pliers or tongs to hold the thermostat and submerge it in the water.
Watch the thermostat closely as the water heats. It should start to open at the temperature specified by the manufacturer (often stamped on the thermostat).
When fully open, the thermostat should remain open as the water temperature rises.
Remove the thermostat and let it cool. It should close again as it cools down.

If the thermostat does not open or close properly, it needs replacement.

Method 2: Testing Thermostat in the Vehicle

What you need:

A temperature gauge or OBD-II scanner (optional but helpful)
Basic tools to remove the radiator cap (only when engine is cool)

Steps:

Start the engine and let it idle.
Watch the temperature gauge on the dashboard. It should rise steadily to the normal operating range.
Carefully feel the upper radiator hose. When the thermostat opens, the hose will suddenly become hot as coolant flows through.
If the hose stays cool or the temperature gauge rises too high without change, the thermostat may be stuck closed.
If the engine temperature stays low for a long time, the thermostat may be stuck open.

Using an OBD-II scanner can provide real-time temperature data for more precise monitoring.

Eye-level view of a mechanic testing a car radiator hose temperature — Mechanic checking temperature of radiator hose during engine warm-up

Tips for Handling Your Thermostat

Always work on a cool engine to avoid burns from hot coolant.
Wear gloves and eye protection when handling coolant or working near the engine.
Replace the thermostat gasket or seal when installing a new thermostat to prevent leaks.
Use the correct thermostat model for your vehicle; check the owner's manual or parts store.
Dispose of old coolant properly according to local regulations.

When to Replace Your Thermostat

If testing shows the thermostat does not open or close at the right temperature, replacement is necessary. Also, if you notice corrosion, cracks, or damage on the thermostat, it’s time to install a new one. A faulty thermostat can cause engine damage if left unchecked.

Replacing a thermostat is a relatively inexpensive repair that can prevent more serious engine problems. If you are not comfortable performing the test or replacement yourself, a trusted mechanic can do it quickly.