I believe the CA trucks will throw a P0380 for a single failed glowplug. Federal trucks take a few more.
I just had injectors replaced under the 200K warranty and I had them do glow plugs at the same time for the P0380 code. They are only about $15 each, but the labor getting to them can hurt if that is the only reason you are in there.
Do a search on LB7 Glow Plugs for a good thread
2003 K3500 Crew Cab, Banks Stinger Pkg, Exhaust Brake & Exhaust, OilGuard Bypass filter, Lube Specialist Secondary Fuel Filter, Allison Deep Pan, TTT Mirrors, B&W Turnover Ball and Companion Hitch, Truck Trunk Sliding Box and V-Box, LINE-X, Co-Pilot Live GPS, Hitchhiker 31.5 LKTG 5th Wheel 28' Wells-Cargo Gooseneck Car Hauler