OBD-II

OBD-II is the second generation of on-board diagnostics, and it is a standard rather than a part: a common socket, a common set of codes, and a common way for an outside tool to ask the computer what it has found. The connector sits inside the car, within reach of the driver's knee on most vehicles, and a technician plugs a scan tool into it to read the fault codes the computer has stored. Because the standard fixes the socket and the code list, one tool reads almost any car sold here since the mid-1990s.

What the port buys is not a diagnosis. A code names the circuit or system the computer is unhappy with, not the failed part — a misfire code tells the technician which cylinder to look at, and the technician still has to test the plug, the coil and the injector to find out why. Clearing the codes does not repair anything either: the monitor runs again, the fault comes back, and the light returns.

The port and the light are two halves of one story. The light says a fault was found; the port is where the code that names it is read.