Hi guys. Excuse me for popping in. GM has a number of tricks designed in to force you to go back to the dealer for most repairs. Computer modules have to be programmed to the vehicle, and the instrument cluster is a computer, but it gets its information from the Body Computer, which was cleverly built into the radio so you can't replace that with an aftermarket one. Communication could be lost intermittently between either of those computers.
Another trick to be aware of is that Body Computer / radio is the master computer that tells all the other computers when to turn on. With the dealer's scanner, the mechanic has the option, (don't ask me why), to press "lock" under the Body Computer menu. There's no "Are you sure?" Message, and once "lock" is pressed, it can not be undone. For the rest of the life of the vehicle, every computer is tied electrically to that Body Computer and will never work in another truck. If the Radio ever needs to be replaced, as they all do since they have a really high failure rate, every other computer must be replaced, then they all must have the software programmed in over an internet connection. That can easily make repairs cost more than the vehicle is worth. Imagine the bill for a Cadillac with up to 46 additional computers.
I've never actually run into a vehicle yet that had the computers locked, but think about what one disgruntled mechanic could do. If they are locked, there's no way to tell when it was done or who did it. The owner wont even know it until the radio quits and he's told how many computers have to be replaced. The only reason I can think of to design a vehicle like this is to sell more parts or send them to the junk yard earlier. I can't guarantee that this locks every computer module, but we have heard so many warnings about this from high-level schools and other mechanics that it is worth bringing up. With the new "CAN BUSS" electrical system, every light switch and sensor is now a computer module, and it's hard to believe they need software installed to make them work.
The point is, this is something the dealer is going to have to diagnose. On older vehicles we could use a scanner to run the gauges up and down from the Body Computer or the instrument cluster to see which one wasn't responding. It's possible the software has to be updated, and the intermittent gauges are the way to get the truck back to the dealership. GM is one of the few manufacturers that will not allow independent shops to download software. Only Hyundai allows anyone anywhere to download anything, including service manuals, for free. Toyota and Chrysler allow anything except Security systems for a fee. No one but the dealer gets anything from GM, and the dealers don't share it for free.
You have the advantage of driving the truck to the dealership with obvious proof everything else is working, and if they accidentally do something to require repairs not related to the gauges, they should be willing to take care of it. They are the only ones who will get help from GM too. Thanks to the insane engineers hanging computers onto every conceivable system that never needed computers before, we're seeing all kinds of weird problems, and probably 25 percent of the time those problems are fixed with software updates. Kinda makes you wonder who will be around to do that for those of us who keep our vehicles for 25 years or who maintain classic cars. I don't think we're going to see 2007 anythings in an old car show 20 years from now.
Tuesday, February 18th, 2014 AT 5:36 PM