Sounds to me like the dealer put his foot in his mouth. You can NEVER guess its a module because most of the time its not. Also Onstar CAN NOT diag anything. All they can do is read a code. They are not the tech. Beacuse of what the service adviser told you they should eat the cost. Next time something like this happens, is advise the customer that they may be liable for the cost of repair if its determined its not a warranty issue like this.
