What is OBD-II?

OBD-II (On-Board Diagnostics, second generation) is a standardized vehicle diagnostic system mandated in all cars and light trucks sold in the United States since 1996. It monitors vehicle emissions and engine performance through sensors and the engine control module (ECM). When a problem is detected, the system stores a Diagnostic Trouble Code (DTC) and illuminates the Check Engine Light on the dashboard. OBD-II standardized the diagnostic connector (a 16-pin DLC), communication protocols, and trouble code formats across all manufacturers.