TEST 1: Thermostat Operation

Checking Coolant Temperature with Scan Tool

Before you jump into the test, on your car or mini-van, please remember that:

1.)  It's critical, for the accuracy of the test (and your safety), that you start out with a cold engine

2.)  As the engine warms up, do not open the radiator cap for any reason.

OK, this is what you need to do to get this show on the road:

  1. 1

    The first step is to check that the cooling system is completely topped off with coolant.

    So, if you have already verified that the engine is completely cold (and by cold I mean that it's at ambient temperature), remove the radiator cap and check the level of the coolant. Add coolant if necessary and re-install the radiator cap.

  2. 2

    Next, check the temperature of the upper radiator hose with you hand. The hose should be cold (at ambient temperature)

    Don't skip this step since you need to make sure that the upper radiator hose is cold (at ambient temperature) before cranking and starting the engine.

    If the hose is warm or hot, let the engine cool down some more and start then.

  3. 3

    Connect your scan tool to the vehicle and get to its Live Data mode.

    Scroll down to the PID for the Coolant Temp Sensor.

    This PID will be labeled: Coolant °F or Coolant °C (see the photo in the image viewer above).

  1. 4

    Crank and start the engine but leave the hood open (the hood must remain open for the entire test).

    The engine will start to warm up and eventually reach it's operating temperature. In the meantime, you need to keep an eye on the coolant temperature that your scan tool is displaying.

    If you started out with an engine at ambient temperature, then your scan tool should report a coolant temperature sensor temperature that should be within 10 degrees of ambient temperature. For example, If it's 95° F outside, then the coolant temperature sensor should report a temp reading between 95 to 105° F.

  2. 5

    As the engine starts to warm up and the coolant starts to absorb this heat, your scan tool will display this increase in temperature

    When your scan tool finally shows 150° Fahrenheit (65° C), lightly touch the upper radiator hose to check its temperature.

    If the thermostat is OK, the hose should still be cold (ambient temperature). In other words, it should be the same temperature you noticed before you started the engine on your car or mini-van. If so, continue to step 6.

    If the hose is hot, this tells you that the thermostat is either missing or stuck open, since at this temperature (150° F) the hose should still be cold to the touch. Replace the thermostat. Also, there's no need to continue on to the next steps.

  3. 6

    As the engine continues to run, the coolant temperature will eventually reach 190° Fahrenheit (87° C). When it does, lightly touch the upper radiator hose once again.

    At this temperature, the thermostat should start to open. Lightly touch the upper radiator hose and check its temperature. It should start to get warm to hot (compared to its temperature at the beginning of the test).

    If the hose does not feel like it's heating up, don't worry about it just yet, continue to the next step.

  4. 7

    Let the engine run till the Scan Tool reports a temperature of 200° F (93° C) and lightly touch the upper radiator hose again.

    The hose should now be hot.

  5. 8

    Alright, whether the upper radiator hose is hot or not, turn the engine off and let's interpret your test results.

Let's take a look at what your results mean:


CASE 1: The radiator hose got hot at 190° F and stayed hot at 200° F. This test result confirms that the thermostat is functioning correctly.

Here's why: The thermostat's job is to prevent coolant from circulating, into the radiator, before a certain temperature. So below 190° F, the upper hose should be cold to the touch. After 190° F, the thermostat opens up and lets the now hot coolant circulate into the radiator.

CASE 2: The radiator hose DID NOT get hot at 190° F nor at 200° F. This test result confirms that the thermostat is BAD and is stuck closed. Replace the thermostat.

CASE 3: The radiator hose got hot below 150° F. This result confirms that the thermostat is BAD and is stuck open. Replace the thermostat.

Here's why: If the thermostat were working correctly, it would not let any coolant circulate within the hose and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to circulate immediately and the hose will feel warm to hot as soon as the engine starts to run.