Onboard Diagnostics II-A second-generation emissions diagnostic system required on all 1996 and newer vehicles.The OBD II system monitors vehicle emissions, and illuminates the Check Engine or Malfunction Indicator Lamp (MIL) if it detects a problem that causes emissions to exceed the federal limits by 50% or more. The OBD II system also stores diagnostic trouble codes to help technicians diagnose the cause of the emissions problem. OBD II cars and light trucks also have a standard diagnostic connector.