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ANSWER by Visitor , March 07, 2010
On the 04 and 05 trailblazer/Envoy 4.2 ltr the check valve is located and attached to the transmission dip stick. Very easy to replace has a bolt to hold on dip stick and two mounting bolts, hose, and plug-in. I would recommend replacing as moisture build up causes rusting and/or electrical failure.
There are two secondary AIR check valves on you 2001 GMC Yukon XL. The valves are locate one on each side of the engine on the pipes that are attached to the exhaust manifold. The check valves cannot be cleaned, they must be replaced when they fail. I would suggest replacing the valves in pairs. Changing only one valve could cause poor flow through the old valve resulting in a fault code for the valve that was not replaced.