Vehicles may also have a CEL or "check engine light" as was common for OBD-I cars, but it is not required.
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However, because this will not happen if the powertrain is operating properly, it also serves to tell the driver when something is wrong with it. The MIL, or malfunction indicator light, is present to tell the driver that their vehicle may be producing emissions greater than 1.5 times the federal limit. In both OBD-I and OBD-II cars the PCM will be constantly running the car either richer if the sensor says the mixture is lean, or vice versa, but OBD-II cars also have a sensor behind the catalytic converter to see if the catalyst is functioning. This voltage-generating sensor produces 450mV in the case of stoichiometric combustion - the state in which all of both the air and fuel are consumed in the reaction. All OBD-II cars have two oxygen sensors, which detect the presence of O2 in the engine exhaust. While OBD-I systems dramatically reduced the quantity of pollutants produced by automobiles as compared to non-computer control systems like mechanical fuel injection or carburetors, the need to further reduce emissions led to this more complicated systems. OBD-II provides a specification for what data must be collected, and how it should be presented to the technician. In the U.S., OBD-II's main job is to make sure that vehicle emissions do not exceed 1.5 times the FTP, or federal test procedure, which measures vehicle emissions.
Computer-controlled cars prior to OBD-II are referred to as OBD-I cars, though there was not actually any standard prior to OBD-II. Some USDM cars went OBD-II as early as 1994. It became mandatory in 1996 for all cars sold within the US to be equipped with an OBD-II system. OBD-II, which stands for On-Board Diagnostics, second revision, is a standard for automotive computer operation and diagnosis.
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