No start 07 gxp po641

[07 black gxp]

Drove the car to work, everything was fine... get in to get food and it starts, idles, then dies... throws reduced engine power and was showing low oil pressure but that's gone now.... it cranks and did fire a few times... when it did fire I had no response when I pressed the has pedal... now it won't fire anymore... any ideas?? I checked the code and saw it is related to the 5 volt reference to the ecm... 5 different things that can cause this... How do I check these?

Thanks for any help

[SDGTP99]

This may help you , http://www.grandprixforums.net/threads/89729-P0641-Code