If something is amiss with your cooling system — like unpredictable spikes on the engine temperature gauge, the heater failing to blow hot air or the engine overheating or not reaching operating temperature — a good place to start troubleshooting is the thermostat.

“Thermostat failure is a very common repair, and one you are likely to see on most vehicles, especially older and higher-mileage ones,” says JulieAnna Petrosky, owner of Southwest Auto.

While thermostats typically last around 80,000 to 100,000 miles, high-quality ones can survive longer. The good news is that on most vehicles, the thermostat is not too hard to access, and if you have a little automotive DIY wherewithal, you should be able to fix the problem yourself.

Here’s how to diagnose and fix a failing car thermostat.

Car Thermostat Overview

A car’s thermostat regulates the flow of coolant through the engine and is an essential part in the operation of your vehicle. In most cases, an overheating or no-heat condition in your vehicle is due to a faulty thermostat. And since the T-stat price is only about $8, it makes more sense to replace it than to spend hours diagnosing the problem. If that doesn’t fix it, you’ve only spent about two hours learning how to replace a thermostat.

Symptoms of a Failing Car Thermostat

Fortunately, a thermostat failure is one of the many car problems your can diagnose and fix yourself. Here are the signs your car thermostat is failing:

  • The temperature gauge reads high and the engine overheats.
  • The temperature changes erratically.
  • The vehicle’s coolant leaks around the thermostat or under the vehicle.

Engine Overheating

Here’s how thermostats work. The manufacturers inject a mixture of ground-up brass and wax into a copper cup called a pellet. Then they slip a highly-polished metal rod through a rubber “O-ring” gasket and into the wax. The pellet is sealed with a crimp ring.

Next, a metal “skirt” is welded around the pellet. When the engine is cold, a spring forces the skirt up against a seat (just like a closed faucet), stopping the flow of coolant. As the engine heats up, the wax melts and expands.

Expansion pressure builds to the point where the wax tries to “spit out” the metal rod. But the rod can’t go anywhere. It’s attached to a “bridge” on the other side of the thermostat.

Eventually the pellet itself moves, overcoming the spring’s pressure. So the thermostat opens and allows coolant to flow. The entire system works well until the metal rod corrodes. That corroded rod damages the rubber seal and the wax leaks out. Once that happens, the thermostat stops opening, coolant stops flowing and your engine overheats. The result can be catastrophic engine failure costing several thousand dollars.

Erratic Temperature Changes

Keep an eye on your dashboard’s temperature gauge. If the needle spikes and drops, a thermostat failure may be to blame.

Coolant Leaking

Check the thermostat housing for leaked fluids. The coolant helps to keep your engine functioning at the right temperature, and the lack of fluid can affect its performance.

How to Test a Car Thermostat

Before removing the thermostat from your vehicle, check the coolant flow and temperature.

Remove the radiator cap and idle the engine. If the coolant flows right away, your thermostat is stuck open. If it is not flowing, wait until your engine warms up. The coolant should warm to the right temperature and begin to flow after 10 to 20 minutes. If it does not start to flow, but the temperature gauge on your dashboard rises, your thermostat is stuck closed. If your coolant flows normally, there may be another cause of your overheating engine.

To check the temperature of your coolant, start with a cool radiator and engine. Idle your engine and use a thermometer to read the engine block or cylinder head temperature. Then, check the temperature of the upper radiator hose. Wait five minutes and test again. Complete the test a total of three times. If the temperature does not rise by much, your thermostat is stuck open. If the radiator hose remains about the same temperature, but the dashboard gauge rises dramatically, your thermostat is stuck closed. If the upper radiator hose rises to about the same temperature as the engine block, your thermostat is allowing the coolant to flow properly. You may have a different problem causing cooling issues.

For the most accurate diagnosis, Alan Gelfand, owner of German Car Depot, recommends removing the thermostat to check whether it’s working properly.

To test the thermostat outside of the car, place it in a pot of water on your kitchen stove and tie a string to it, so it can’t touch the bottom of the pot.

“Heat the water at a slow pace to observe the thermostat behavior,” says Gelfand. “The device should fully open as the water temperature continues to rise.”

Use a thermometer to note the temperature at which it begins to open when it finishes opening completely. Then remove it from the pot and observe it as it cools, to make sure it closes gradually. Your owner’s manual will tell you the range where it should open and close, plus the thermostat’s operating range is often stamped on the thermostat’s surface.

Replacing the Thermostat

If your thermostat fails, don’t just replace it and top off the coolant reservoir. It’s sending you a message of a more serious problem — the corrosion inhibitors in your coolant also failed. So flush the system and add fresh coolant any time you replace the thermostat.

And don’t try to save a few dollars by buying a cheap thermostat (about $5). Premium thermostats are built to resist corrosion, plus have stronger springs and more precise temperature response.

“DIYers are often looking to cut costs, so buying a budget thermostat is a common mistake,” says Petrosky. But, “cheap thermostats are an especially poor idea because inaccurate function can lead to overheating and engine damage. OEM or reputable aftermarket brands are worth the extra money.”

FAQ

How long does it take to replace a thermostat?

It depends on where the thermostat is located in your car (how accessible it is) and your expertise, but usually between 30 minutes to a couple of hours.

“Professional labor time reaches between three and four hours for replacing complex German luxury car thermostats that need disassembly to access the component,” says Gelfand. Also, don’t forget to factor in the time it will take to properly bleed the cooling system after replacing the thermostat, he says.

What happens if I ignore a bad thermostat?

If a thermostat is stuck open, the vehicle’s engine will run cool, which can cause incomplete fuel combustion, resulting in carbon buildup and decreased gas mileage. Over time, this can damage the engine.

If the thermostat is stuck closed, that is more serious, as it can cause the engine to overheat, resulting in severe consequences like cylinder head warping and cracking, head gasket failure and even the engine seizing up.

“On most vehicles the fix for this will cost well into four figures and can get up into five figures on some luxury vehicles,” say Petrosky. “In short, a very small problem can turn into a very large problem.”

About the Experts

  • JulieAnna Petrosky has over fifteen years of experience in the automotive repair industry, specializing in European vehicles. She owns and operates Southwest Auto, a 15,000-square-foot advanced automotive facility in Dallas, Texas.
  • Alan Gelfand has been the owner of German Car Depot for more than 30 years. His business is an independent automotive service center in Hollywood, Florida, specializing in the service and repair of German vehicles.