How can I tell whether a mechanical thermostat works as it should?

Is there a test that can definitively tell me if a thermostat is good or bad?


2 Answers 2


Typical Thermostat

Failure Modes

Mechanical thermostats fail in one of two ways:

  1. Stuck-closed

    The pintle/spring doesn't retract to permit flow at the set-point temperature that is usually stamped on the thermostat itself. which can lead to the engine running at hotter than expected.

    If hot enough, this can lead to serious issues such as a compromised head gasket or warped cylinder head.

  2. Stuck-open

    The pintle/spring doesn't actuate to maximize flow restriction at temperatures cooler than the set-point temperature. This usually results in the engine running at cooler than designed-for temperatures.

    Running an engine too cool can result in issues with the emissions and catalytic converter efficiency due to increased hydrocarbons in the exhaust.

Test Procedure

The thermostat needs to be out of the car and on the bench for this.

The temperature at which the thermostat opens is usually stamped on the thermostat itself. In the absence of such, a technical manual or parts supplier will require referencing.

Appropriate PPE is strongly recommended as this test involves boiling hot water.

  • Place the thermostat in boiling water¹:

    • if the thermostat opens, this confirms the thermostat is able to open
  • Remove the thermostat from boiling water²:

    • if the thermostat returns to the closed position, this confirms the thermostat is able to close

¹ - remember that the temperature of boiling water varies with altitude; it's 100 °C at sea level

² - now what did I say about PPE again?

Why engine coolant temperature won't give the full story

Just because your engine temperature gauge isn't where it should be doesn't mean that the thermostat is bad.

Here is a list of other plausible reasons that could cause the engine temperature gauge to read too cool or too hot:

  • faulty engine coolant temperature sensor
  • environmental conditions
  • radiator fans not turning on (too hot)
  • radiator fans running when they shouldn't be (too cold)
  • blockage in the coolant piping (too hot)
  • compromised water pump (too hot)
  • leaks (too hot)
  • 3
    "Running an engine too cool can result in issues with the emissions and catalytic converter efficiency due to increased hydrocarbons in the exhaust." I'd be more concerned about oil viscosity and engine wear. But yeah, I had this problem a few months ago. I just put the thermostat in a pot on the stove with a thermometer to see if anything changed at the "stamped on" temperature. It got to 100°C and still didn't open so I swapped it out for a new one.
    – voices
    Commented Dec 25, 2015 at 12:05
  • 1
    @zaid That's a beautiful answer. Commented Dec 25, 2015 at 15:49
  • 2
    @DucatiKiller thanks! I hope this helps many, many people
    – Zaid
    Commented Dec 25, 2015 at 18:49
  • 2
    Really like your answer. It looks like it's from a professional publishing company. :)
    – Ppoggio
    Commented Dec 26, 2015 at 18:52
  • It should be noted that some manufacturers do not recommend testing a thermostat in boiling water. For example Ford: NOTE: If no damage is found during the inspection, do not attempt to open the thermostat using hot water or other heat sources. This method is not an accurate means to test the function of the thermostat and may damage the thermostat.
    – Ives
    Commented Jan 16, 2017 at 10:54

Just ran into a variation of the stuck open failure. Thermostat opened and closed at the correct temperature (195 F) when tested in a pot of hot water. However, when installed, the thermostat would allow water to pass at temperatures 40 F cooler. Replacing the thermostat fixed the problem.

Edit for dlu: I can read the coolant temperature on the vehicle info center. With the old Tstat the upper hose would get warm at about 150 F indicating coolant flow. It didn't get much higher while driving in 30 F weather. After checking the old Tstat operation while out of the car, I put it back in with the same results. The new Tstat holds 192 F.

  • Welcome! Testing rules! How did you determine that it was working differently when it was installed than it did when you tested it?
    – dlu
    Commented Jan 15, 2017 at 17:23

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