How To Test The Thermostat (2002-2009 4.2L Chevrolet TrailBlazer, GMC Envoy)

This tutorial will explain how to test the coolant thermostat with a scan tool in 'live data' mode.

All of the test steps are explained in detail. So you'll quickly and easily determine if the thermostat is good or is stuck closed or is stuck open.

APPLIES TO: This tutorial applies to the following vehicles:

• 4.2L Chevrolet TrailBlazer: 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009.
• 4.2L GMC Envoy: 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009.

Important Suggestions And Tips

TIP 1: You'll need a scan tool to test the thermostat with the info in this tutorial. If you don't have a scan tool, check out my recommendation:

• Actron CP9580 Scan Tool.

TIP 2: Don't open the radiator cap with a hot engine. If you need to add coolant (or water) to the radiator, do it with a cold engine.

TIP 3: Start your diagnostic with a completely cold engine to guarantee an accurate test result from the thermostat test described in this tutorial.

TIP 4: Take all necessary safety precautions when working around a running engine.

Where To Buy The Thermostat And Save

The following engine coolant thermostats are of known automotive part manufacturers (no knock-offs):

Symptoms Of A Bad Thermostat

The thermostat in your 4.2L Chevrolet TrailBlazer (GMC Envoy) will fail in one of two ways:

• It will stay stuck in its open position and let coolant circulate all of the time.
• It will remain stuck closed and not let the engine coolant circulate at all.

When the thermostat is stuck open, you'll see the following symptoms:

• Bad gas mileage.
• If it's wintertime, the heater will not work.
• If your vehicle is equipped with a temperature gauge (and not just a temperature light), it will always show you a low reading or none at all.
• Won't pass the state emission test.

Most folks are usually surprised to find out that a stuck open or missing thermostat wreaks such havoc with gas mileage, but it's true.

When the thermostat is stuck closed, you'll see the following symptoms:

• The engine will overheat.
• Cooling fans come on, but the engine does not cool down.
• If your vehicle is equipped with a temperature gauge, it will max out.
• If your vehicle is equipped with a temperature light, it will shine nice and bright to let you know that an engine meltdown is occurring.

OK, let's get testing.

Thermostat Performance Test

All right, this is the test you signed up for!

To get going, please keep in mind that the accuracy of the test depends on starting with a cold engine.

Also, as the engine warms up, do not open the radiator cap for any reason.

NOTE: The OEM thermostat temperature is 192° F.

Let's get started:

PART 1:

1. 1

   Open the hood.

2. 2

   Check the engine coolant level. Top it off if necessary.
   
   CAUTION: The engine must be cold.

3. 3

   Check the temperature of the lower radiator hose. You can use your hand.
   
   The hose should be at ambient temperature. If the hose is hot, let the engine cool down completely.

4. 4

   Connect your scan tool to the vehicle and turn the key to the ON position. Don't crank or start the engine.

5. 5

   Go to your scan tool's Live Data mode.
   
   Scroll down to the PID for the coolant temp sensor. On most scan tools, this PID will say: Coolant °F or Coolant °C.

6. 6

   You should see a coolant temperature sensor temperature within 10 degrees of ambient temperature on your scan tool at this point.

PART 2:

1. 7

   Start the engine. Monitor the engine coolant temp on your scan tool as the engine runs.

2. 8

   Turn off the engine when the coolant temperature reaches 150° Fahrenheit (65° C) on your scan tool.

3. 9

   Check the temperature of the lower radiator hose.
   
   The lower radiator hose should still be at the ambient temperature you noticed in step 3. If so, continue to step 9.
   
   If the hose is hot, you can conclude the thermostat is stuck open or missing. Replace the thermostat.

4. 10

   Restart the engine and continue monitoring the coolant temp on your scan tool.

5. 11

   Turn the engine off when the coolant temperature reaches 200° Fahrenheit (93° C) on your scan tool.

6. 12

   Check the temperature of the lower radiator hose.
   
   The lower radiator hose should now be hot.

7. 13

   Turn the engine off.

Let's take a look at what your test result means:

CASE 1: The lower radiator hose got hot at 200° F. This confirms that the thermostat is functioning correctly.

Here's why: If the hose was hot at 200° F, the thermostat opened and let the coolant (which has been absorbing the engine's temperature) circulate to the radiator.

CASE 2: The lower radiator hose WAS NOT hot at 200° F. This confirms that the thermostat is bad and is stuck closed. Replace the thermostat.

CASE 3: The lower radiator hose was hot at 150° F (or less). This 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 cooler than 190° F 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 has been running for a few minutes.