What is the difference between OBD-I and OBD-II?

OBD-I was the first generation introduced in 1981, with each manufacturer using their own proprietary system and connector. OBD-II, mandated from 1996, standardized the diagnostic connector (16-pin DLC), communication protocols, and trouble code formats across all manufacturers. OBD-II provides more comprehensive monitoring, standardized DTCs (P, B, C, U codes), and a universal scan tool interface. While OBD-I required manufacturer-specific tools, any standard OBD-II scanner works on any compliant vehicle.