I started getting check engine (goes into limp mode; 1/2 power) with my C-7 a few months ago. Dealer wanted $175 each time to read codes so I bought a Silverleaf system for my laptop; $310. Works great.
You also get pretty good tech support diagnosing codes from Silver leaf.
It told me I was getting either high pressure or no signal from the turbo boost sensor. I have it scheduled for a full service next week and the tech said he could test that one and/or replace it if its the cheap one.
Cross my fingers.