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What is OBD-II?

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  • Written By: J. Dellaporta
  • Edited By: Niki Foster
  • Last Modified Date: 24 September 2016
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OBD-II is a system used in automobiles to monitor various components of the vehicle, detect malfunctions, and store the information in the vehicle’s on-board computer to be recovered later by a service technician. OBD-II is an acronym for on-board diagnostics; the “II” denotes the second and most current version of this technology. Beginning in the late 1970s, vehicles sold in the United States have been equipped with electronics to control various systems and diagnose malfunctions with the goal of minimizing pollution. This came about in response to Congress passing the Clean Air Act and establishing the Environmental Protection Agency (EPA) in 1970. These electronics varied between manufacturers and model years, making the retrieval of diagnostic information potentially costly and time-consuming.

In 1988, the EPA and California’s Air Resources Board (CARB) mandated that vehicle manufacturers include self-diagnosing programs to ensure that their emissions equipment would remain effective for the vehicle’s service life. The Society of Automotive Engineers standardized a connector plug and a set of diagnostic test signals. Upon equipment failure, this system illuminated a malfunction indicator light (MIL) on the vehicle’s dashboard, often called the “check engine” light. This system, required in all 1991 and newer automobiles, became known as on-board diagnostics I, or OBD-I.

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CARB studies soon concluded that OBD-I systems would not detect emissions components unless they completely failed, and that in some cases the vehicle could still pass an emissions test. New laws and requirements went into effect on 1 January 1996 - the standard adopted for OBD-II. Every vehicle built for sale in the U.S. from that date is equipped with OBD-II.

OBD-II uses various sensors throughout the car to provide the computer, also called an electronic control module (ECM) with information such as engine and ambient temperatures, vehicle speed, and so on. The ECM then advances or retards ignition timing and adds or subtracts fuel accordingly. It also tests the signals of all attached sensors. When a signal is missing or out of spec, the OBD-II system illuminates the MIL and stores a corresponding diagnostic trouble code its memory.

The information from the OBD-II memory is read through a connector inside the auto. OBD-II improves on OBD-I not only in its more sophisticated diagnostic abilities, but also in that it allows three types of data to be read: trouble codes, real-time data - the raw sensor information reported to the OBD-II computer, and freeze-frame data - a “snapshot” of sensor data at the moment the car’s MIL went on. OBD-II codes are read using cables and software built to communicate with OBD-II systems. These can take the form of stand-alone units or software that is installed on a PC. Some are complex models intended for professional technicians; simpler units are priced towards hobbyists.

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anon126291
Post 2

I like your article. However, Why do the auto companies hide these codes. I have a 98 Dodge which displays the code on the odometer when you turn the ignition on fast three times (Pxxxx). The codes are in the ECU, so why make people buy extra tools? Toyota could have used this. Well, maybe not. This seems like another come to the dealer method.

anon23025
Post 1

I just wanted to say thank you for this informative article. I'm trying to learn more about my Integra and most Honda/Integra forums are designed for more advanced users who are far more knowledgeable (they usually modify their cars, often by themselves). So it's nice to have a place to come to for the basics without feeling like an idiot! Nice job.

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