Simply put, OBD, or On Board Diagnostics, is the set of sensor and microprocessors on your car that make the Idiot Lights go on. For those of you with non-ADD attention spans, let's go over some history.
About Two Score and Seven Years Ago...
Throughout the 1960's, American car manufacturers were building some of the most badass rides ever created. And by "badass" I mean powerful. Ungodly power. Besides having no fuel economy, there were no emissions standards and the cars burned leaded gasoline. The Great Tree Hugging kicked off in 1966 when California required cars to meet certain emissions standards. Two years later, the federal government followed suit. In 1970, the United States government passed The Clean Air Act and formed the Environmental Protection Agency.
Car makers began to incorporate more and more electronics into their cars in order to produce lower emissions engines. Remembers the Volkswagen Beetle? Those bastards were immortal; some of them drove for over 1,000,000 miles. But the cost of adapting them to the new regulations raised their (extremely low) price to the point where they were no longer worth importing into the American market. But I digress . . .
Hook Me Up
Through the 70's and 80's, more and more electronics and sensors popped up in American automobiles. Engines began getting electronically controlled fuel feed and ignitions systems to meet emission standards. In turns out that the sensors used to monitor the fuel and ignition systems could also provide diagnostic data. The first standard was OBD-I. It was implemented to help control emissions by requiring annual emissions checks, but failed due to the non-standardization between manufacturers' systems. The next, and current standard, is OBD-II. Every car made after January 1, 1996 for sale in America has an OBD-II system. The harnesses used to connect the scanning or reading equipment to the car is the same now, as opposed to different harnesses for each manufacturer during the OBD-I days. While the phyiscal connection is standardized throughout the industry, different manufacturers adhere to different ISO standards which equate to different voltages and signals on different pins in the harness. Cars outside the US have an OBD-II CAN standard, which American vehicles must implement by 2008.
A Light at the End of the Tunnel
Nowadays, an overwhelming amount of information can be gained from your OBD system. All manner of real time data can be collected from a running engine, but none of it is interesting to non-gearheads. We'll skip that for now. What can be very informative is the non-real time data. The ODB systems record past malfunctions, as well as current malfunctions.
The only time we regularly interact with the OBD system is by ignoring the idiot light. Technically speaking, it's the malfunction indicator light (MIL). Again, it can go off for dozens of reasons. Once you've driven around for a few weeks with the light on, you feel guilty enough to pull into a garage somewhere and check it out. Once you regain consciousness from the shock of the Diagnostic Examination, you come to your senses and leave. Drive straight to your local parts store and buy an OBD scanner for your car. OBD-II is easy to lay hands on, and all you OBD-I'ers out there, like me, can find scanners as well if you ask the clerk. I got mine for $40, but whatever it costs, pay it. Even if it's $100, it pays for itself if you use it once. Alternatively, if you're near an Advance Auto Parts, they will do a free scan for you.
Assuming you bought the scanner, go into the parking lot, read the simple instructions and follow them exactly. You will get a read out of Diagnostic Trouble Codes (DTC), which are commonly just referred to as "codes." March right back up to the parts counter and query the oldest clerk you can find. If you're DIY, he'll line you up with everything you need. Congratulations, you're on your way to becoming a techno-shade tree mechanic. Just remember to clear those saved codes after you work on the problem; the procedure will be covered in the scanner's documentation. Otherwise, you'll go crazy wondering why it's taken 9 hours to fix your thottle position sensor. If you're mechanically challenged, you at least know what's wrong with your car and you can mitigate being bent over the counter at the repair shop.