Serial Data Reference
WARNING: This page is about a different car, the 2007 Cadillac SRX. However, it is still accessible from the selected car via links, so may be relevant.
The scan tool communicates over the various busses on the vehicle. In order to avert misdiagnoses of No Communication with a specific module and to determine if a module is low or high speed, refer to Data Link References for a list of modules, the busses they communicate with, and the RPO codes for a specific module.