I think you need to go to another dealer, I had the same code.My gm dealer said glow plugs are not covered under the engine 100k warranty. So I bought one at car quest.
So far I disconnected the metal intake tube right after the turbo down to the air cooler radiator. Disconnected the 2 main plugs for the engine. Then was able to get to the #5 cylinder (I hope they are number like a short block 305) I pulled out the "U" clip and disconnected the one plug that hooks onto the glowplug like a motorcycle's spark plug. Then unhooked the plug that is horizontal.
All fine and dandy right?! Ok now how in the world do you get off that big bolt/harness/plug/etc. "thingy" that screws down around the the glow plug?!
Took it to the dealer, they popped a new glow plug in and I had it back in about 2 hours. Its definitely covered by warranty.