Immobilizer Description and Operation: Notes

The immobilizer system functions are provided by the body control module (BCM) and the engine control module (ECM), as well as any modules which store and report the security code.

When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the encrypted code in the key is energized by the immobilizer coil surrounding the ignition lock cylinder. This immobilizer coil is part of the immobilizer antenna. The encrypted code transmits a signal that contains its unique value, which is received by the BCM. The BCM then compares this value to a value stored in memory. The BCM also monitors various modules to determine that the stored environment ID matches.

If both the environment ID and the value received from the transponder match, the BCM will send the prerelease password via serial data to the ECM. If the encrypted codes unique value is incorrect or the environment ID does not match, the BCM will send the fuel disable message to the ECM.

When the ECM receives the BCM prerelease password, the ECM will challenge the password. The ECM sends this challenge back to the BCM via serial data. Both the ECM and BCM perform a calculation on this challenge. If the calculated response from the BCM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

The components of the theft system are as follows:

• BCM
• ECM
• Immobilizer antenna
• Ignition key/fob
• Security indicator
• Various modules which store and report the environment ID are BCM, IPC, HVAC, EBCM, ECM and SDM

BCM is programmed with the environment ID at the manufacturer.