My '02 D/A sat for a month without being moved after I had heart surgery. When I was able to drive again, the truck would not move in any gear, and the service engine soon light was on. I sat and ran the engine at 1,500 RPM for about 10 minutes with the AT in neutral, and the transmission then would shift into any gear. I phoned my local Chevy dealer, and the diagnosis was a faulty NSBU switch on the Allison AT. The switch is not sealed properly and allows water to corrode the wiring. This is a known problem for the 2001-2003 Allison AT's. GM issued a TSB on this problem, and even though my truck is past warranty the dealers replaced the switch as a courtesy warranty. The new improved switch is water tight to prevent re-occcurance. I hope my experience helps.