A common problem on 2004.5-2006 Duramax LLY engines is what's typically known as harness rub. The injector wiring harness is zip-tied to a metal flange next to the alternator. The problem is that the harness loom stops too short, leaving just a thin layer of protective material between the metal flange and the wires inside the harness. Over time, vibrations cause the metal to rub through the thin layer and into the wires' insulation, eventually exposing the wires themselves. The best preventative fix is to cut the zip tie and cover the harness with more wire loom or a chunk of rubber hose-basically anything that can act as a guard between metal and wire-and zip tie it securely in place against the metal flange.