How To Test The Thermostat (GM 3.8L V6)

In this article, I'll show you how to test the thermostat, to see if it's stuck open or closed, without removing it from the engine.

As you might already be aware, when the thermostat fails... it does one of two things... either the engine overheats or stays so cool that the heater will not work (if it's winter time that is).

Thankfully, there is a way to test the thermostat to find out if it has failed or not, without having to replace it first.

Also, since all of the 3.8L V6 equipped vehicles that belong to the GM line (Chevy, Buick, Olds, Pontiac) use an electric fan motor to cool the engine down... I've included a simple test to see if the fans are coming on at the appropriate temperature.

To help you navigate this article, here are its main points:

  1. Important Suggestions And Tips
  2. Symptoms Of A BAD Thermostat
  3. TEST 1: Thermostat Operation
  4. TEST 2: Electric Fan Motor Operation
  5. Engine Thermostat Basics
  6. Other Tips And Suggestions
  7. Where To Buy The Thermostat and Save
  8. More Test Articles

En Español You can find this tutorial in Spanish here: Cómo Verificar el Termostato del Motor (3.8L V6 GM) (at:

Important Suggestions And Tips

TIP 1: The fastest, easiest and safest way to check that the thermostat is good or BAD is with a scan tool. This scan tool has to have Live Data capability... since a simple code reader will not do.

You don't need the GM factory scan tool or an expensive professional technician level scan tool. A generic scan tool will do great and I've written this article with this tool in mind (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

TIP 2: Testing the thermostat involves checking its operation with the engine at its normal operating temperature... and so you have to be careful. Do not, under any circumstances, open and/or remove the radiator cap with a hot engine.

Opening and/or removing the radiator cap with a warmed up or hot engine can cause you severe burns. Your safety is your responsibility... be careful, use common sense and think safety all of the time.

TIP 3: To successfully test the thermostat, it's very important that you start your troubleshooting with a completely cold engine. If the engine in your vehicle is hot or has been running for an extended amount of time... let it cool down completely. This is important for safety reasons and for the accuracy of the thermostat test.

Symptoms Of A BAD Thermostat

The thermostat in your 3.8L equipped GM car or minivan will fail in one of two ways: It will either stay stuck at its open position and let coolant circulate all of the time or it will stay stuck closed. When it stays stuck closed... your car or mini-van will overheat.

Here are some more specifics:

CASE 1: If the thermostat is stuck open, you'll see the following symptoms:

  1. BAD gas mileage.
  2. If it's winter time, the heater will not work.
  3. 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.
  4. Won't pass the state emission's 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.

CASE 2: If the thermostat is stuck closed, you'll see the following symptoms:

  1. The engine will overheat.
  2. Cooling fans come on, but the engine does not cool down.
  3. If your vehicle is equipped with a temperature gauge, it will max out.
  4. 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...