2004 gmc envoy xlt wont start security light on

Diagnostic Starting Point - Theft Deterrent
Begin the system diagnosis with the Diagnostic System Check - Theft Deterrent . The Diagnostic System Check will provide the following information:
• The identification of the control module(s) which command the system.
• The ability of the control module(s) to communicate through the serial data circuit.
• The identification of any stored diagnostic trouble codes (DTCs) and their status.
The use of the Diagnostic System Check will identify the correct procedure for diagnosing the system and where the procedure is located.
your best bet is have your vehicle towed to the dealer or some other qualified repair shop .