You have an issue with the Passlock immobilizer. It requires a Tech2 scan tool to be able to diagnose these but one of the more common problems is the sensor in the ignition switch messing up. I'm giving you the relearn procedure that may or may not work. If it doesn't or if it happens frequently, you will have to get it diagnosed properly.
This guide can help
PASSLOCK 10 MINUTE RELEARN
Time required, approximately 10 minutes:
1. Insure that the battery is fully charged.
2. Use the scan tool in order to clear the DTCs.
3. Turn the ignition switch from the OFF position to the CRANK position attempting to start the vehicle. The vehicle will start and then stall.
4. Leave the ignition switch in the ON position while observing the security indicator.
5. When the security indicator turns off, which can take up to 10 minutes, turn the ignition switch off. Wait 10 seconds.
6. The BCM and the PCM will learn the new code on the next start attempt.
Monday, January 10th, 2011 AT 12:58 PM