I had a P1516 code and Reduced Engine Power on my Yukon XL 5.3L 2003... Cleaning the TB didn't help. It would go into limp mode and after sitting for a few hours would recover. I replaced the TB three weeks ago with a new one and everything seems fine so far. The electronics in the TB has to produce a certain amount of wattage to the computer (TCM). If it doesn't fall into a specific range, it will produce the limp mode..