On Board Diagnostics (OBDII) and the MIL

This primer document will attempt to explain the basic concepts of the On Board Diagnostics (OBDII) standard and the function of the MIL (Malfunction Indicator Lamp) on your vehicle’s dashboard. For more detailed information go to the links provided in Sources
The first generation of On-Board Diagnostic requirements, called OBD I, was developed by the California Air Resources Board (ARB) and implemented in 1988. As technology and the desire to expand On-Board Diagnostic capability increased, a second-generation of On-Board Diagnostics requirements was developed. This second version of On-Board Diagnostic capabilities is called "OBD II". The Clean Air Act Amendments of 1990 mandated that, beginning with the 1996 model year, all light-duty vehicle and trucks made available for sale outside of the state of California must also be equipped with OBDII. In addition, EPA also requires that medium duty vehicles up to 14,000 pounds must also be equipped with OBD II systems beginning in the 2004 model year. In the future, EPA expects that all heavy-duty vehicles over 14,000 pounds will eventually be equipped with OBDII systems.
The OBD II 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, and assists repair technicians in diagnosing and fixing problems with the computerized engine controls. If a problem is detected, the OBD II system illuminates a warning lamp on the vehicle instrument panel to alert the driver. This warning lamp typically contains the phrase Check Engine or Service Engine Soon. The system will also store important information about the detected malfunction so that a repair technician can accurately find and fix the problem.

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