Inside OBD-II

All cars from 1996 on have an OBD-II connector so that owners and mechanics can figure out what's happening under the hood. The National OBD Clearing House offers an online database showing the connector location in most cars. Even though the connector has 16 pins, only 5 are used. The connector pulls error codes from the engine's central computer, making it the 21st-century replacement for the Dwell meter, the timing light, and the gap gauge. There are four different protocols that the different automakers use, but modern scanners can easily work with any car sold in the last decade.

Each code has a unique letter and four numbers that correspond to a specific fault.

The codes are derived from the dozens of sensors that every car has for telling everything from wheel speed to the voltage of the electrical system. Each code has a unique letter and four numbers that correspond to a specific fault. The letter can be a B (body), C (chassis), U (network), or P (power train). Practically speaking, just about every code you'll have to deal with will have a P prefix, but the proliferation of electronics means that the others may come up. The code is actually simple to figure out. The first number is used by the manufacturer for specialty codes, while the next one differentiates specific problems. The final two digits denote what subsection is under scrutiny. So P0781 shows a problem with the transmission getting into second gear, and a P0087 indicates low fuel pressure.

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