There are 2 security system, namely anti-theft and immobilizer.
Here are the 2 systems.
DESCRIPTION & OPERATION
Anti-theft system is controlled by Body Control Module (BCM). BCM receives signals from hood switch, trunk switch, door switches and Local Control Modules (LCMs). There are 5 LCMs located throughout vehicle.
To set anti-theft system in armed mode, remove ignition key, close all doors, trunk and hood. Lock doors using key or remote controller. The SECURITY light will illuminate for 30 seconds, then flash in 2.4 second intervals, indicating system is in armed mode.
To cancel armed mode, unlock and open any door or trunk lid using key or remote controller. SECURITY light will flash every second, indicating system is disarmed.
Alarm will activate (horn sounds, headlights flash and starter circuit is disabled) if any of the following conditions occur while system is in armed mode: Engine hood or trunk lid is opened using the hood or trunk lid release.
Door(s) is unlocked without key or remote controller.
Tampering with any door/trunk lid key cylinders.
To deactivate anti-theft system, unlock door or trunk using key or remote controller.
If it is the immobilizer that is preventing the engine from being started, ensure you have the correct key and the transpnoder is intact. Quit often keys are dropped and if it breaks open, the internals would scatter all over. When this occurs, there is a tendency for the transponder to be lost.