Q: Anti-theft system issues after leaving key in overnight. Car won't start.

Hello. From what you describe it would appear that your security system may be activated and needs to be reset. This likely happened as a result of leaving the keys in the vehicle overnight. The procedure for resetting the security system is as follows. If this procedure does not reset the system then it may be possible that the system will need to be reset by a GM factory scan tool, requiring a visit to the dealership.

Reset procedure for GM Passkey III Security Systems:

1. Insert the master key with the black head into the ignition lock cylinder

2. Turn the key to ON but do not start the engine. The SECURITY light will turn on and stay on.

3. Wait 10 minutes or until the SECURITY light to turn off.

4. Turn the key to OFF for 5 seconds.

5. Turn the key to ON but do not start the engine. The SECURITY light will turn on and stay on.

6. Wait 10 minutes or until the SECURITY light to turn off.

7. Turn the key to OFF for 5 seconds.

8. Turn the key to ON but do not start the engine. The SECURITY light will turn on and stay on.

9. Wait 10 minutes or until the SECURITY light to turn off.

10. Turn the key to OFF. The key transponder will be learned on the next start cycle.

11. Start the vehicle. If it starts and runs normally, the relearn is complete. If you need to program additional keys,

12. Turn the key to OFF.

13. Insert the next key. Turn it to ON within 10 seconds of removing the previously used key.

14. Wait for the SECURITY light to turn off. That may happen quickly. You may not even notice it.

15. Repeat steps 12 through 14 for all additional keys.