Sensor Applications & Service Procedures
	OBD I & II Systems
	OBD I Systems: In the late 1970s On-Board Diagnostic systems were produced by the automakers. These first generation systems used either one or two O2 sensors depending on the exhaust configuration. The ECMs were programmed to monitor sensor operating parameters and to set diagnostic codes if the sensor failed. A hard code meant the sensor signal was out of range or there was no sensor activity.
	OBD II Systems: Beginning in the mid 1990s a second generation of On-Board Diagnostic systems were introduced. Among other requirements, OBD II systems must monitor the performance of the catalytic converter. The solution was to add additional O2 sensors before after the catalytic converter. Depending on the number of cylinders and the exhaust pipe configuration, you can find 2, 3, 4 or 6 sensors being used.