Hoping someone can help me, I have a 03 5.9 cummins and it will not start, I've checked wires, tried a new switch commuter, and the actual ignition switch, as well as a brand new starter, and it still will not even turn over, I've tried putting it in neutral. Still nothing, does anyone have any idea what it may be?? Also check the neutral safety switch, any help will be truly helpful thanks is advance

 

What is your battery voltage? Clean all your grounds and battery connections! As a far shot on the firewall drivers side there is a yellow connector that goes to the battery if that is disconnected will not crank. It's a push together wire YELLOW CONNECTOR. Did you try and jump the starter?

 

Thanks for the reply, I've checked the wire running to the starter and it was nearly 13 volts I will check for the yellow wire in a few, I have jumped the starter and it does start.

 

Might try the starter relay,
Here is a schematic for your truck starting system. hope this helps

starting-charging-starting-circuit-1-of-1 2003 chevy.pdf