Many injector issues can actually be traced to the injector wiring harness that is part of the valve cover gasket on the Common Rail Cummins. A simple continuity test can determine if your injector(s) isn't firing properly due to the harness. Simply unplug each end and using a multimeter, or other testing equipment, determine what the amperage drop is. If the drop is 0.5 or under, that is within spec, but if it is over, then you need to replace the harness. This kit from Dorman is a great replacement for the factory piece.