Service ABS System

You may have a wheel speed sensor that is acting up in your 2005 Pontiac Grand Prix. Your erratic braking can be explained as follows: When the light is off the ABS system will be functioning, erratic wheel speed signals to the ABS module will cause erratic operation of the ABS. When the light is on, a fault has been detected and the ABS system is off. The ABS will does not work at all when the light is on but the regular braking system will function normally. A fault code should be stored in the ABS control module which can be of help diagnosing your problem.