GM Service Manual Online
For 1990-2009 cars only

The EBCM performs system self-diagnostics, and can detect and often isolate system malfunctions. When a malfunction is detected, the EBCM sets a Diagnostic Trouble Code (DTC). The DTC represents the malfunction, turns on the ABS and/or TRAC OFF indicators (in almost all instances if equipped), and may disable the ABS/TCS functions as necessary for the duration of the ignition cycle.