Editing On-board diagnostics (section)

===OBD-I===

The regulatory intent of OBD-I was to encourage auto manufacturers to design reliable [[emission control system]]s that remain effective for the vehicle's "useful life".<ref>{{citation |url=https://www.moreycorp.com/obd-i-obd-ii-a-history-of-on-board-diagnostics/ |title=OBD-I to OBD-II: A History of On-Board Diagnostics |date=December 21, 2022 |publisher=The Morey Corporation |location=US |access-date=2023-03-23}}</ref> The hope was that by forcing annual emissions testing for [[California]] starting in 1988, <ref>{{cite web |url=https://ww2.arb.ca.gov/our-work/programs/obd |title=OBD - On-Board Diagnostic Program |publisher=[[California Air Resources Board]] |location=US |access-date=2024-06-02}}</ref> and denying registration to vehicles that did not pass, drivers would tend to purchase vehicles that would more reliably pass the test. OBD-I was largely unsuccessful, as the means of reporting emissions-specific diagnostic information was not standardized. Technical difficulties with obtaining standardized and reliable emissions information from all vehicles led to an inability to implement the annual testing program effectively.<ref>{{cite web |url=https://ww2.arb.ca.gov/resources/fact-sheets/board-diagnostic-ii-obd-ii-systems-fact-sheet |title=On-Board Diagnostic II (OBD II) Systems Fact Sheet |publisher=[[California Air Resources Board]] |location=US |date=2019-09-19 |access-date=2024-06-02}}</ref>

The Diagnostic Trouble Codes (DTC's) of OBD-I vehicles can usually be found without an expensive scan tool. Each manufacturer used their own Diagnostic Link Connector (DLC), DLC location, DTC definitions, and procedure to read the DTC's from the vehicle. DTC's from OBD-I cars are often read through the blinking patterns of the 'Check Engine Light' (CEL) or 'Service Engine Soon' (SES) light. By connecting certain pins of the diagnostic connector, the 'Check Engine' light will blink out a two-digit number that corresponds to a specific error condition. The DTC's of some OBD-I cars are interpreted in different ways, however. Cadillac fuel-injected vehicles are equipped with actual ''onboard'' diagnostics, providing trouble codes, actuator tests and sensor data through the new digital Electronic Climate Control display.

Holding down 'Off' and 'Warmer' for several seconds activates the diagnostic mode without the need for an external scan tool. Some Honda engine computers are equipped with [[Light-emitting diode|LEDs]] that light up in a specific pattern to indicate the DTC. General Motors, some 1989–1995 Ford vehicles (DCL), and some 1989–1995 Toyota/Lexus vehicles have a live sensor data stream available; however, many other OBD-I equipped vehicles do not. OBD-I vehicles have fewer DTC's available than OBD-II equipped vehicles.