Diagnostic Starting Point

Diagnostic Chart:

Begin the diagnosis of the data link communications by performing the on-board diagnostic system check. The diagnostic system check will direct you to the correct procedure within the data link communications section, when a communication malfunction is present.

Establish serial data communications with each controller, read and record DTCs from each controller. It is important to systematically approach serial data communications diagnostics. This vehicle has two separate data links with three different circuits to communicate to the various data links.

The high speed CAN link which supports real-time information between the ECM, TCM, EBTCM, and BCM is accessible through two different keyword circuits.

1. Circuit 2960 supports communication from DLC to ECM and TCM.
2. Circuit 5043 supports communication from DLC to EBTCM.

The other modules communicate on the Class II link which is configured in a ring, except for the OnStar(R) module. The ring configuration provides redundant communication paths, so that a single open does not prevent a serial communications malfunction.