I would like to know where the thermostat is on a Chevrolet Trailblazer 4.2 liter engine?

The thermostat on a Chevrolet Trailblazer with a 4.2L engine is located on the engine, at the back of the engine, near the firewall. It's often tucked away and difficult to access. You'll likely need to remove some components to reach it, possibly including some plastic engine covers and potentially the upper radiator hose.

It's not a simple "look and find" location. A repair manual specific to your year and model Trailblazer is highly recommended before attempting to locate and replace it. The manual will have diagrams and instructions to help you access it safely and efficiently.

Without knowing the exact year of your Trailblazer, I can't give more precise instructions. The engine bay layout can vary slightly between model years.