Where are the spark plugs located on Vortec 4200 gmc envoy engine?

The Vortec 4200 (4.2L I6) engine in a GMC Envoy has its spark plugs located on the top of the engine. To be more precise, they are nestled between the valve covers. You'll need to remove parts of the intake system (possibly the upper intake manifold) to access them, depending on the specific year and configuration of your Envoy. A repair manual specific to your vehicle's year will give you the most accurate and detailed instructions.