Signs of a Bad Thermostat, and How to Fix It

Stephen Fogel
March 1, 2018


Signs of a bad thermostat

When your thermostat does not respond properly to the temperature conditions in your cooling system, you can have a problem. The most noticeable issues occur when the thermostat gets stuck in either the open or closed position. A malfunction can result in a trouble code, generated by the engine’s computer, which can turn on your check engine light.  

If the thermostat is stuck open

If your thermostat gets stuck in the open position, it’ll allow full, unrestricted circulation of the coolant. You might see these issues as a result:

  • Temperature gauge reads lower than normal
  • Heater doesn’t work
  • Automatic transmission has trouble shifting into higher gears
  • Fuel economy drops noticeably

These are all byproducts of an engine that is running too cool. It results in all sorts of inefficiencies within your vehicle. While this situation may not be life-threatening to your car, your comfort and fuel bills will suffer until you get it fixed. 

If the thermostat is stuck closed

Here’s what can happen to your car’s health when your thermostat sticks closed, blocking off the movement of your coolant through the radiator:

  • Temperature gauge goes into the red
  • Temperature warning light comes on
  • Steam or smoke comes from under the hood
  • Engine overheats
  • Head gasket fails 
  • Severe engine damage

A permanently closed thermostat can be deadly to your engine. It’s important to heed the warning signs, which can save you some very costly repairs.

Other thermostat problems

The thermostat’s location atop the engine, directly in the coolant flow, places it in a hostile environment. Here are some of the other problems that can befall your thermostat:

  • Rust
  • Corrosion
  • Deposits
  • Leakage from its mounting
  • Electrical or electronic malfunction (for map-controlled thermostats)


How does a thermostat work?

Your thermostat keeps the car's coolant at the temperature that’s optimal for your engine. The thermostat helps your engine warm up quickly on a cold morning and helps prevent overheating.

The thermostat in your car is a valve that controls the flow of coolant between the engine and the radiator. A heat-sensitive element controls the operation of this valve. It stays closed when your coolant is cold, letting it get warmed more quickly by the heat of the engine. The thermostat opens when the coolant is hot enough, allowing it to flow through the radiator and give up its heat.

Some newer vehicles have electronic thermostats. These are known as map-controlled thermostats. They have the ability to open and close, as well as to blend hot coolant with cold in response to commands from the engine computer. This finite temperature control can dramatically improve performance. The benefits include faster warm-up, higher performance, better emissions control and improved fuel economy. 

Fixing a bad thermostat

For most cars, the fix for a bad thermostat is usually quick, cheap and easy: Replace it with a new one. Be aware that the price can creep up for some vehicles, though. 

When replacing a thermostat, make sure that it is correctly installed, using new gaskets to eliminate the possibility of leaks. If you’re not comfortable dealing with this repair, let your mechanic handle it. It's a good practice to replace the thermostat whenever the cooling system is serviced or opened for any reason, like replacing the water pump or radiator. 

If your vehicle has a map-controlled thermostat, with a trouble code thrown by the engine computer, track down the cause with a code reader. Plug it into the OBD-II port and run a scan to see what the problem is. If the cause is the thermostat, replacement is the solution.

The relative ease and inexpense of this repair goes out the window if you ignore the signs of a stuck-closed thermostat, though. If your engine starts to overheat, immediately pull off the road, shut off your car and call your mechanic. This can save your engine, which could cost you thousands of dollars to repair or replace.

Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.

No comments yet…

Sign in to comment

Related Questions

See what others have asked about this, or visit the Questions page to ask your own question.
Where is thermostat and how to replace?
Trying to get my hands on detailed instructions on how to replace my thermostat. i've got the general idea but not...