Just to share, sometimes the "fix" is something totally unrelated. My 2007 5.3 silverado for over two years have had the service engine soon light illuminated. I use the " fixed" reader plugged into my port to easily pull the codes and reset the light. For two years the codes I have were P0191 and P069e, fuel rail pressure circut range/ performance and fuel pump control module. All this time the truck ran fine. I would reset the codes but after an hour or so they would always reappear. Since the truck ran good I just didn't worry about them. I also noticed my alternator output would vary from 14.9 V to 12.5 V drifting back and fourth slowly.
Decided to flip my battery around just to make it easier to access and replaced both battery cable ends at the battery. The original ones felt tight but there was some slight oxidation on the posts. So with the new clamps I polished all the surfaces up, coated all contact surfaces with dielectric grease and tightened everything down. It's been two weeks now and the service engine light has never returned! Also my alternator output stays at 14.5 Volts. Truck ran good before but seems even smoother now. As an added bonus my MPG has increased from 11.5 MPG to 15 MPG! Sometimes the solution is the simplest possibility.