OBD-II codes refer to specific malfunctions of a vehicle's engine or emission control systems and are produced by the on-board diagnostic system with which all modern vehicles are equipped. These codes are read by a scanner connected to a communication port located in the engine compartment or the passenger cabin.
After the Clean Air Act was passed by the United States Congress in 1970, automobile manufacturers began to utilize electronic engine controls to reduce emissions as well as to provide feedback regarding failures or malfunctions of the system. The first version of these codes and a standard connector plug for the diagnostic scanner were developed by the Society of Automotive Engineers in 1988. OBD-II is an expanded version of these codes adopted by the Environmental Protection Agency for use beginning Jan. 1, 1996.