This is a very old thread that I have found in a few different forums with no answers or solutions. I had the exact same problem on my 2004 LB7. Check engine/no codes. My problem ended up being a bad wire (Black) in the lower section of the under hood fuse panel. Either worn from rubbing or a mouse chewing on it. I also had a bad fuse on Tap 1. I corrected these and was then able to read the codes.
Hope this helps future searches.
