Related Terms • OBD-II

Definition: Computerized engine control systems have a certain amount of built-in self-diagnostic capability to detect problems that affect engine, transmission performance as well as the emissions. The same is true for the antilock brake system and other onboard systems that are computer controlled. When a fault is detected, the computer will store a diagnostic trouble code in its memory and illuminate the "Check Engine" light. On some vehicles, the computer can be put into a special diagnostic mode by grounding certain terminals on a diagnostic connector. This will cause the Check Engine or other lights to flash out the fault code. On many vehicles, though, a scan tool must be plugged into the computer system to access and read the codes.