Body Control Module (BCM)
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
IMPORTANT:
On some vehicles, if the immobilizer system is unable to read the ignition key transponder value after the vehicle has started, the immobilizer system will consider itself malfunctioning. The immobilizer system will enter a fail enable state. When the immobilizer system is in a fail enable state, the vehicle will NOT stall or stop running. If the immobilizer system is in a fail enable state when the ignition is switched OFF, the immobilizer system will remain fail enable until it is able to read a learned ignition key transponder value. When the immobilizer system is in a fail enable state the immobilizer system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.
When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the body control module (BCM) supplies battery voltage to the theft deterrent exciter module. The transponder embedded in the head of the key is energized by the theft deterrent exciter module which is surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains its unique value, which is received by the theft deterrent exciter module. The BCM monitors the theft deterrent exciter module for the transponder value. The BCM then compares this value to a value stored in memory, learned key code. The BCM then performs one of the following functions:
- If the transponder value is correct, the BCM will send the fuel enable password to the ECM/PCM via serial data.
- If the transponders value is incorrect, the BCM will send the fuel disable password to the ECM/PCM via serial data.
- If the BCM is unable to measure the ignition key transponder value for one second due to a damaged or missing transponder or a damaged theft deterrent exciter module, the BCM will send the fuel disable password to the ECM/PCM via serial data.