Have you checked your PCM for codes ?
I would do that long before just randomly changing out parts on guesses.
Do not go buy new UVCH until you have pulled your VC to see if they need to be changed. Before even that go pull your IDM plug. If you do not have one buy or borrow a basic multimeter and check all the pin outs. Follow these testing procedures:
http://www.gbreman.com/articles/?p=242
As has been said confirm with that same multimeter that the GPR is actually good.
But first thing is pull those codes. IF you do not have a code reader see if someone else does that can pull them or you can borrow it.
Honestly anyone that works on their own computer controlled vehicle IMO should have AE especially a diesel like the 7.3