Here is the how to from the GM service manual that I just downloaded. READ THE LAST PART
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30-Minute Immobilizer Relearn
This 30-minute relearn procedure will relearn the start enable password between the theft deterrent module (TDM) and the engine control module (ECM).
With a master vehicle key, turn ON the ignition, with the engine OFF.
Observe the security indicator. After approximately 10 minutes, the telltale will turn OFF.
Turn OFF the ignition, and wait 5 seconds.
Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes.
Important: The vehicle learns the password on the ignition switch transition from OFF to RUN. You must turn the ignition OFF before attempting to start the vehicle.
With a master vehicle key, start the vehicle. The vehicle has now learned the key transponder information and/or the ECM has now learned the fuel continue password.
If additional keys are required to be learned, refer to Adding Keys .
With a scan tool, clear any DTCs.