Joe...
You are in Chandler I see. Hop on AZGMPerformance.com or is it GMAZPerformance.com and find PDXGTP, aka a local GP guy in Chandler. He should be able to fix this up quickly. Thomas is a good guy who will know exactly what you need to do to the pcm to pass.
Since you have an 05, it's highly unlikely any Magnaflow is going to cure the code. Most high flows don't meet the specs the pcm is looking for.
Here's 97-03 drive cycle. May also be the same for you.
http://www.grandprixforums.net/how-r...cle-50998.html