My 06 3500 Cummins is pretty much stock but on the 160000 miles approximately it's been having some difficulty starting every morning 5-7 seconds of starting everytime after waiting for the start light before cranking and even after it's warmed up it still takes 2-3 seconds to fire. Once it starts it's OK more or less. I do know I have one number 3 fuel injector or solenoid that misfires at idle sometimes, but it is getting less often and coding less often. Batteries are just over 4 years old but they are putting 12.6-8 V. I have 12.4 volts from the relay after multiple uses of the prestart. I cleaned all the terminal connections with no change. I have a continuity check through the heater grids but that is not tested individually. Temperatures in the morning 30 to 40 degrees in N. NV.