OBD II systems monitor vehicle conditions and components that are related to vehicle emissions, such as the catalyst in the catalytic converter, engine misfire, the engine coolant temperature, and the oxygen sensor. Federal law requires that an OBD testing program be implemented in all states no later than January 2001, to ensure that emissions control equipment is operating correctly. When a malfunction or deterioration of an emissions control component occurs which may cause an increase in the vehicle's emissions, the driver is notified by a warning light. The early warning allows the owner to have the problem repaired before the vehicle has emitted significant excessive emissions. By alerting the driver to the need for potential repair, vehicles will be properly serviced long before emissions become a problem.
Early repair can also often prevent further damage to the vehicle's emission control system.
The OBD system can detect a malfunction or deterioration of the various components, often before the driver becomes aware of the problem. Early detection and repair of malfunctions may prevent more significant problems, and thus more costly repairs, later. For example, a poorly performing spark plug can cause the engine to misfire, a condition sometimes unnoticed by the driver. This engine misfire can, in turn, quickly degrade the performance of the catalytic converter and permanently damage the catalyst. With OBD detection of the engine misfire, the driver would be faced with a relatively inexpensive spark plug repair.