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Bad Thermostat Symptoms

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Bad thermostat symptoms can manifest in different ways:

  • Engine overheating
  • Overheating in heavy traffic
  • Overheating while idling for a long time
  • Overheating while driving at speed
  • Erratic overheating
  • Slightly overheating all the time
  • Heater not working properly
  • Engine overcooling

Keep in mind that many faults, other than a bad thermostat, can lead to an overheating engine. So you need to verify that your thermostat has actually failed.

On the other hand, overcooling is always caused by a failed thermostat, unless your car's cooling fan is running nonstop while the engine is running. In this case, check the fan clutch or a possible shorted fan switch, depending on your particular model.

On 2002 and newer models, you'll become aware of an overcooling problem because the car's computer will set a diagnostic trouble code (DTC) P0128. The computer monitors the time it takes the cylinder head to reach operating temperature. If the time is out of range within two consecutive cycles, the check engine light will come on.

The following sections will help you find out whether your thermostat has failed.

It is a good idea to have the repair manual for your particular vehicle make and model. The manual helps you locate the thermostat, and how to go about testing it and, if necessary, how to replace it.

You can buy a relatively inexpensive repair manual through Amazon.

Haynes manuals include:

  • Photographs
  • Step-by-step procedures
  • Parts location and replacement
  • Systems description
  • Diagnostics
  • Maintenance schedule
  • Electrical diagrams

So you can recoup your small investment in a short time.

In This Article

1. How a Thermostat Works
2. Engine Overcooling
3. Why an Engine Overheats?
4. A Visual Inspection
5. Checking Thermostat Operation
5.1. Checking Thermostat Operation Through the Radiator Neck
5.2. Checking Thermostat Operation Using Your Hand
5.3. Checking Thermostat Operation With a Thermometer
VIDEO: Checking the thermostat Without Removing It From Your Car
5.4 Testing a Thermostat Outside the Engine
6. Can I Run the Engine Without a Thermostat?
7. Resources

1. How a Thermostat Works

The thermostat's job is to sense engine coolant temperature, open the valve when coolant has reached a predetermined temperature, and control coolant flow into the radiator.

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On most vehicles, you can locate the thermostat housing by following the upper radiator hose to the engine. The hose connects to the housing on the engine side.

Most thermostats come with a temperature number stamped on their body. This is the thermostat operating (opening) temperature. Usually, you'll find thermostats rated at 180F or 195F (82C and 91C).

If you don't know your thermostat's rating, look it up in your repair manual. This number is important when performing certain tests, and to make sure you get the correct unit for your application.

How a Thermostat Fails

Four different ways a car's thermostat may fail


Stays open

Engine overcooling

Stays close

Engine overheating

Remains partially open

Engine overcooling

Operates in a skewed way and out of range

Engine overcooling, overheating or both

2. Engine Overcooling

An overcooling condition is easier to diagnose.

First, make sure that the cooling fan is not running continuously.

  • If your vehicle is equipped with a fan clutch, a stuck clutch will cause the fan to run all the time.
  • A shorted fan switch will cause a cooling fan to run continuously as well.

If the cooling fan is working fine, replace the thermostat.

3. Why an Engine Overheats?

Depending on your particular model, an engine overheating will cause the temperature gauge to read too hot, a warning light to come on, or coolant and steam to come out of the overflow tank.

Certainly, a stuck close or partially close thermostat will raise engine temperature beyond the recommended level. However, other faults can also cause an engine to overheat.

Other common sources of engine overheating include:

  • Low coolant level
  • Loose fan belt (depending on vehicle model)
  • Faulty cooling fan
  • Loose water pump belt
  • Faulty water pump
  • Bad lower radiator hose
  • Clogged radiator or coolant passages in the engine
  • Retarded ignition timing

Always check or test the thermostat before blaming a thermostat for an overheating engine. The following two sections will help you in this case.

4. A Visual Inspection

When your engine is overheating, a careful visual inspection can help reveal problems with the cooling system.

Check the following:

  • Check for a loose or missing fan belt.
  • Verify coolant level.
  • Check for coolant leaks, if dealing with frequent low coolant levels.
  • Ensure the front of the radiator is not covered with debris, preventing air flow.
  • Check the appearance of engine oil that may reveal coolant leaks (milky color).
  • Use a mechanic's stethoscope, or small piece of rubber hose, to check for water pump noises that may reveal a faulty pump.
  • Check for air bubbles in the coolant with the engine running (radiator cap removed), that can reveal combustion leakage.

5. Checking Thermostat Operation

You can use a few methods to check thermostat operation at home.

Some methods are simple and do not require removing the thermostat or special tools, and take only a few minutes.

Other methods require a thermometer or removal of the thermostat.

You can use one or more of the four methods described below to check your thermostat.

5.1. Checking Thermostat Operation Through the Radiator Neck

If your car radiator has a neck, you can use this simple method to verify thermostat operation.

  1. With the car's engine completely cool, remove the radiator cap.
  2. Start the engine and verify that antifreeze is not flowing by looking through the radiator neck. If it is flowing, the thermostat is stuck open, and you need to replace it.
  3. Wait a few minutes for the engine to reach operating temperature, perhaps 15-20 minutes. Then the antifreeze should start flowing. If it doesn't flow, the thermostat is stuck close, and you need to replace it.

5.2. Checking Thermostat Operation Using Your Hand

This is a simple and quick manual test you can do yourself at home.

  1. Pop the hood open and locate both the lower and upper radiator hoses.
  2. Start the engine and touch with your hands the upper and lower radiator hoses, so you can check and compare their temperature. Be careful and keep your hands and body away from moving parts, though.
  3. After about 15 to 20 minutes (or before, if the temperature gauge enters the red zone), touch the upper and lower radiator hoses again. If the hoses are not hot to the touch, the thermostat is not opening, causing the engine to overheat.

5.3. Checking Thermostat Operation With a Thermometer

Another simple way to check thermostat operation is with the use of a digital or infrared thermometer.

  1. Pop the hood open and ensure the engine is cool.
  2. Take the engine's temperature with the thermometer touching the engine block near the thermostat housing. Also, take the temperature at the upper radiator hose that connects to the thermostat.
  3. After about 15 or 20 minutes, take the temperature again at the previous two points.
    • If the hose maintains the same temperature as before, or close to it, the thermostat is stuck closed (the engine is overheating).
    • If the hose rises in temperature but does not reach engine operating temperature, the thermostat is stuck or is not working properly. In either case, the thermostat must be replaced.

If you need to replace the thermostat, consult your repair manual. The Resources section at the end of this article also includes links to other articles that will help as well.

The following video shows you some tests done on an engine with a stuck-open thermostat using an infrared thermometer. It serves as a visual reference in case you want to check your own thermostat by hand or using a thermometer as well.

Checking the Thermostat Without Removing It From Your Car

5.4 Testing a Thermostat Outside the Engine

You can remove the thermostat from the engine and test it yourself at home. To remove the thermostat, consult your repair manual, or the Resources section at the bottom of this post.

  1. Place the thermostat in a glass container so that it is submerged completely in water.
  2. Hold the thermostat with a piece of rope or wire so that you can hold the thermostat without letting it touch the bottom.
  3. Place a kitchen thermometer in the container without letting it touch the bottom. You only want to measure water temperature.
  4. Heat the water and make a note at what temperature the thermostat begins to open. It should be within 5F (or 4C) degrees of its rating. Compare your reading to specifications.
  5. Remove the thermostat from the container. The thermostat should begin to close soon after.

If the thermostat doesn't run properly, replace it.

6. Can I Run the Engine Without a Thermostat?

Some vehicle owners remove a failed thermostat and decide to operate the engine without one for a while or for a long time.

However, this has negative consequences:

  • The engine operates below its intended temperature.
  • It promotes condensation in the crankcase.
  • Sludge begins to build up.
  • Acid from the sludge will corrode internal engine components.
  • Engine performance will suffer.

In the long run, it actually costs you a lot of money to run your engine without a thermostat.


This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 Dan Ferrell

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