I am sorry that you have not been helped yet. I just came online and saw your question
Is there an aftermarket alarm or remote start system on your vehicle or is the alarm/antitheft/starting system all factory
When it will not start is the security light on steadily or blinking on and off?
If you have not tried the 30 minute relearn system, we need to do that first
30minute GM relearn procedure:
1-TurnON the ignition,with the engine OFF.
2-Attemptto start the engine,then release the key to ON (vehicle will not start).
3-Observe the SECURITY telltale(this is the security light), after approximately 10minutes the telltale will turn OFF.
4-Turn OFF the ignition, and wait 5 seconds .
5-Repeat steps 1through 4 two more times for a total of 3 cycles/30 minutes(the vehicle is now ready to relearn the Passlock(TM) Sensor DataCode and/or passwords on the next ignition switch transition from OFFto CRANK). IMPORTANT: The vehicle learns the Passlock(TM)Sensor Data Code and/or password ***** the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
6-Start the engine(the vehicle has now learned the Passlock(TM) Sensor Data Code and/or password).