This Preliminary Information communication provides information to the technician on how to diagnose and repair vehicles that may have an OnStar Turn-By-Turn Navigation the system that is inoperative. Technician verifies there are no Diagnostic Trouble Codes are present in the Antilock Braking System and Vehicle Communication Interface Module of the vehicle. If no Diagnostic Trouble Codes found call OnStar using the Blue button on the OnStar key pad and ask the advisor for a turn by turn route. Also make a OnStar personal call and a call into the vehicle. If normal diagnostics do not lead to a resolution technician will need to call General Motors Technical Assistance Center for further assistance.