I'd go see the parts changer that put a $400 dollar part on my engine that still has the same problem and demand my money back. Just because a code sets does not mean a part is defective, it simply means the computer has seen a problem on that particular circuit. More testing is needed.
Cant help you with the diagnostics (at home, info at work) but if it is a egr code (and gm has trouble with them) go ahead and drive it until you can get it fixed.
