Hi. My mother owns a 2000 Deville and has been having problems with the message system. She is constantly getting service engine and servie theft system messages. She is also getting the service engine light. She has taken it in to get serviced numerous times to a couple of different Cadillac dealerships and they say there isn't a fault code and they can't find the problem. She is looking to sell the vehicle soon and I'd like to solve this problem before putting it up for sale. I originally thought it had to do with the key she was using. I thought one of her keys was damaged and the pass key system was faulting. But it does this with all three of her keys including the valet key. Any help?