DTC 12: Diagnostic System Check: Description
WARNING: This page is about a different car, the 2002 Pontiac Firebird and 2002 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.
DTC 12 is always displayed as first DTC, indicating Body Control Module (BCM) is in diagnostic mode and DTC display is in progress. If DTCs do not display, perform TESTING . If DTC's display, perform appropriate test in accordance with DTC displayed. See DIAGNOSTIC TROUBLE CODE DEFINITIONS under SELF-DIAGNOSTIC SYSTEM.