05 GMC Envoy PO410 code.

Have replaced relay for air pump and air pump works now. Reset code and a day later code comes back on. I can hear the pump running if I shut the vehicle off right away. What else could it be?

I would bet that the switching valve is stuck closed.

Agreed.

You can unplug it and hook a power wire to one terminal [of the switching valve and another jumper to ground on the other terminal. Make sure you are doing this on the switching valve! You should hear a click when you supply and remove power. With power supplied and the air supply hose from the pump disconnected from the valve you can start the vehicle. You should hear/feel exhaust gas coming out of the tube the air supply connects to. If you don't than replace the valve.

Great thanks. Where is this switch valve located. Pass. side of the engine?

passenger side right above the exhaust manifold towards the rear. You'll see the big fresh air hose going to it.

Found it. Thank you!
Would you happen to know the part number for this?