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What is OBD-II?
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To combat its smog problem the State of California started requiring emission control systems on cars in the early '60s. The US federal government first extended and expanded a version of California's controls nationwide in 1968, and have been enhancing them ever since. Emissions are currently as low as 5% of 1967 levels, depending on what chemical you measure. Canada has mostly tagged along with the US Environmental Protection Agency (EPA), starting with the 1972 model year, and current Canadian vehicles generally meet current US federal emissions limits. Oddly, the Canadian-market 1999 Toyota Tercel, which was never sold in the US (and can't be sold there in any case because it lacks a passenger air bag), has a California-certified emissions system.

Europe has its own emissions standards and systems, which may or may not coincide with North American standards. I do not know how many European cars embody EPA-compliant OBD systems. Some European countries and many non-European countries still do not have substantial emissions control systems, and many of those still use leaded gas.

The US Congress passed the first Clean Air Act in 1970, which established the federal EPA. Over the next few decades, this body set up a series of graduated emission standards, along with a 1975 requirement that all cars eventually use catalytic converters (Honda's famed CVCC system bit the dust because of that). To meet these standards, manufacturers (and the Society of Automotive Engineers, or SAE) eventually turned to electronically controlled fuel and ignition systems. Sensors measured engine performance and a computer (ECU) gathering data from the sensors was used to control the engine's fuel injection system to maximize performance while producing minimum measured pollution. The ECU could also be accessed by diagnostic equipment for fault finding.

In 1982, the federal EPA mandated that all new cars sold in the US come equipped with an engine monitoring system that depended on "feedback" engine controls, and which would illuminate a dashboard lamp if problems arose with the monitoring system. The system allowed auto manufacturers to establish their own communications protocols, interfaces, and error code number definitions. This is the standard where you have to count blinks from an LED, or from the Check Engine light, or turn the ignition key on and off a certain number of times, etc. It does not require a "code reader" as such, unless you count a bent paper clip as a "reader".

The State of California took this system a step further in 1985, decreeing that a more detailed system be installed beginning with 1988 model-year cars and light trucks sold in California. This new system was known as On-Board Diagnostics, or OBD. For use with OBD, the SAE developed a standard connector plug (the Diagnostic Link Connector, or DLC) and set of diagnostic test signals. California's EPA adopted the SAE's OBD programs and recommendations essentially as-is. In 1996, California and the SAE enhanced OBD once more, and this became On-Board Diagnostics, Generation Two, or OBD-II, first required federally for vehicles assembled January 1, 1996.

1994 and 1995 may be considered changeover years. Cars from these years may have an OBD-II-style DLC, but still not necessarily be fully OBD-II compliant. In addition, some Honda models around 1996 retained the old Check-Engine light or LED-flash error identification system in addition to the new OBD-II code system.

More background on OBD history is here: http://www.autoshop101.com/forms/h46.pdf

Part of the mandated OBD-II specification was a set of Diagnostic Trouble Codes (DTC's, or "error codes") that are identical across all manufacturers and models sold after 1996, and are supposed to be readable with any OBD-II reader. They start with a letter and have four numbers after the letter (such as P0300). Automakers were allowed to create their own "enhanced" codes to supplement the required ones. The first number can be anything other than zero, and can cover anything from ABS to airbags to interior climate control.
What those numbers and letters mean (for Model Years 1996-2003)is here: http://www.tegger.com/hondafaq/misc/066000_065578.pdf

Starting in 2003, automakers began a mandated switchover to a new communications standard, Controller Access Network, or CAN for short. By 2008, all new vehicles sold in North America will have switched over to CAN. If you want your OBD-II reader to be usable for other cars you may buy in the future, make sure your reader is CAN-compliant. As of the 2004 model year, no Hondas currently use CAN, but they will soon.

Much more detail about OBD-II can be found here: http://www.obdii.com/
AutoTap also has some good information on emissions control systems.

Last updated: March 30/08