Help I have a 1996 Peterbilt with a 550hp CAT motor - 5EK75924 I keep getting a flash code on the dashboard of 21 which is an ECM problem not enough power/voltage to computer. I have tried everything and can't get this to go off. I have brought it to Peterbilt and another truck repair and nobody can fix it. Does anyone know of a good/solid mechanic in NJ that can help me out?

first make sure all the wires going to the batteris are clean and the fuses on those wires arent corroded. this sometimes will cure the problem