If your LML / LGH Duramax will not start, one of the early things to check is the fuel injector return line pressure which must be a minimum of 3 Bar (44 PSI). If the pressure is low and there are no leaks in the fuel return hose assembly, the most likely cause is the return line pressure regulator which is part of the return hose assembly. The piezo injectors utilized in these engines require back pressure in order to function correctly and the return line constant pressure regulator is designed to maintain between 4 and 11 bar (58 - 160 PSI) in the system. Hard starting is often caused by a regulator that is weak.