Air Injection Valve
GMC Chevy Secondary Air Injection
There is a pattern emissions failure on Chevy Trailblazers and GMC Envoys with the 4.2 engine. The Emissions fault of P0410 Secondary Air Injection system is active. What happens most often is this valve shown above gets stuck due to corrosion and the secondary air from the electric air pump can not flow. Replace this valve and blow the water out of the hoses with compressed air. This secondary air injection valve is available at the GM dealer or an AC-Delco parts supplier.
Once in a while you see an air pump that's not working but 90 percent of the time its just this valve.
They sell the gasket separately so pick one of those up while you're at it.
Clear the engine computer and let the truck sit for 8 hours after repair. Start the truck and let it idle 3 minutes then drive 40 mph for 2 minutes to set the readiness monitor for this system.