California Air Resources Board

On-Board Diagnostic (OBD) systems are self-diagnostic systems incorporated into the computers of vehicles.  The OBD system monitors virtually every component that can affect the emission performance of the vehicle to ensure that the vehicle remains as clean as possible over its entire life.  If a problem with an emissions-related component is detected, the OBD system illuminates a warning lamp on the vehicle instrument panel to alert the driver.  The system will also store important information about the detected malfunction so that a repair technician can accurately find and fix the problem.  Nearly all 1996 and newer vehicles less than 14,000 lbs. (e.g., passenger cars, pickup trucks, sport utility vehicles) are equipped with OBD II systems, which are California's second generation of light- and medium-duty OBD requirements.  Heavy-duty vehicles over 14,000 lbs. have also been equipped with OBD (heavy-duty OBD or HD OBD) systems starting with the 2010 model year.  Many states currently use the OBD system as the basis for passing and failing vehicles in their inspection and maintenance programs (e.g., California’s Smog Check program).  HD OBD is also expected to be incorporated as part of any heavy-duty inspection and maintenance program that is developed in the future.  CARB develops the OBD system regulations for vehicles sold in California and certifies these OBD systems to ensure they are meeting these OBD system requirements.