2002 GMC Yukon XL 2500. Engine size = 8.1 Replaced crank & cam sensors. Question: Starter will not spin or engage. When I turn key to run position all normal dash lights illuminate, fuel pump runs 15 sec. or so. Continue to turn key to start dash lights go out, but no crank.

Starter relay cycles. Switched relay. Fuses all look good under hood & dash. Tried to start in neutral, park & all other gears. Also tried when engine was cold sat over night. No warning just quit.

Answer: Assuming that you are sure the battery is good, sounds like you have a bad starter. Batteries can be tricky and seem to be OK, but not have enough amps to turn the starter. Just to make sure, you can check the purple wire at the starter solenoid, and when you turn the key to crank, that wire should light a test light and have power. If it does, you have a bad starter. If it doesn't have power, you either have a wiring problem, park / neutral switch, non-factory alarm problem, etc, and would need to be checked out. Since you describe the relay clicking, and everything else powering up with the key in 'run', I would bet on a starter.