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1975: Datsun 280Z On-board computers begin appearing on consumer vehicles, largely motivated by their need for real-time tuning of fuel injection systems. Simple OBD implementations appear, though there is no standardization in what is monitored or how it is reported.
1980: General Motors implements a proprietary interface and protocol for testing of the Engine Control Module (ECM) on the vehicle assembly line. The 'assembly line diagnostic link' (ALDL) protocol communicates at 160 bit/s Implemented on California vehicles for the 1980 model year, and the rest of the United States in 1981. The only available function for the owner is "Blinky Codes" that transmit the Diagnostic Trouble Code (DTC) with the blinking pattern of the "Check Engine" (MIL) light.
1986: An upgraded version of the ALDL protocol appears which communicates at 8192 bit/s with half-duplex UART signaling. This protocol is defined in GM XDE-5024B.
1988: The Society of Automotive Engineers (SAE) recommends a standardized diagnostic connector and set of diagnostic test signals.
1991: The California Air Resources Board (CARB) requires that all new vehicles sold in California in 1991 and newer vehicles have some basic OBD capability. These requirements are generally referred to as "OBD-I", though this name is not applied until the introduction of OBD-II. The data link connector and its position are not standardized, nor is the data protocol.
~1994: Motivated by a desire for a statewide emissions testing program, the CARB issues the OBD-II specification and mandates that it be adopted for all cars sold in California starting in model year 1996 (see CCR Title 13 Section 1968.1 and 40 CFR Part 86 Section 86.094). The DTCs and connector suggested by the SAE are incorporated into this specification.
1996: The OBD-II specification is made mandatory for all cars manufactured in the United States to be sold in the United States.


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