Powertrain Control Module
WARNING: This page is about a different car, the 2002 Buick Regal and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
The Powertrain Control Module (PCM) communicates with the Body Control Module (BCM) over the Class 2 serial data line. When the BCM enters tamper mode, it sends a Class 2 message to the PCM in order to disable the starting and fuel delivery systems. If the BCM receives a valid coded key, the BCM sends a class 2 message to the PCM in order to enable the starting and fuel delivery systems. The PCM then allows the vehicle to start.