Ok first problem with this truck for me. 2006 K3500 D/A 56k miles. Got an SES light, (no limp mode) just before getting home from a two week trip with fifth wheel.

Checked for codes and there are none. Checked under hood for low levels on fluids, loose wires, anything obvious and turned up nothing. Oil change is 75% and fuel filter is fresh 12/17/15.

Other than a better scanner, what will set SES light and not leave a code?