The immobilizer system functions are provided by the body control module (BCM). When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the 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, the learned key code. If the value is correct, the BCM sends the fuel enable password via serial data to the engine control module/powertrain control module (ECM/PCM). If the transponders value is incorrect the BCM will send the fuel disable password to the ECM/PCM via the serial data circuit. The components of the immobilizer system are as follows:

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:

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 via the security system sensor signal circuit.

The theft deterrent exciter module uses the following inputs, security system sensor supply voltage and ground circuit. The theft deterrent exciter module uses the following outputs, security system sensor signal.

Important: On some vehicles, if the ECM/PCM is unable to communicate with the immobilizer system after the vehicle has started, the ECM/PCM will consider the immobilizer system to be malfunctioning. The ECM/PCM will enter a fail enable state. When the ECM/PCM is in a fail enable state the vehicle will NOT stall or stop running. If the ECM/PCM is in a fail enable state when the ignition is switched OFF, the ECM/PCM will remain fail enable until communications with the immobilizer system has been restored. When the ECM/PCM 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.

The ECM/PCM verifies that the password received from the body control module (BCM) via the serial data circuit is correct. The PCM can learn only one fuel continue password. If the fuel continue password is correct, the PCM enables the starting and fuel delivery systems.

The ECM/PCM disables the starting and fuel delivery systems if any of the following conditions occur:

The ignition key is a typical looking ignition key with a transponder located in the plastic head of the key. The transponder value is fixed and unable to be changed. The immobilizer system uses the ignition key transponder value to determine if a valid ignition key is being used to start the vehicle. There are approximately three trillion possible transponder values. There are no visible electrical contacts. The keys may be identified by the letters PK3 stamped into the steel shank of the key. The immobilizer system uses the following types of ignition keys:

Master Keys

Master keys have a black plastic head for full access operation of the vehicle. Master keys may perform the following functions:

Valet Keys

Important: Valet keys are NOT standard equipment on all GM vehicle lines.

Valet keys have a gray plastic head and are for restricted operation of the vehicle. Valet keys may perform the following functions: