Diagnostic Aids

WARNING: This page is about a different car, the 2001 GMC Safari and 2001 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.

BCM detects that ignition is ON and sends an appropriate power mode message to other modules. Therefore BCM must be connected to DLC for any other module to communicate with scan tool.
When class 2 serial data circuit is shorted to battery negative voltage, U1300 will be set. See DTC U1300: CLASS 2 SERIAL DATA LINK LOW under DIAGNOSTIC TESTS.
When class 2 serial data circuit is shorted to battery positive voltage, U1301 will be set. See DTC U1301: CLASS 2 SERIAL DATA LINK HIGH under DIAGNOSTIC TESTS.