OBD-II
On-board vehicle diagnostics standard (US since 1996, EU since 2001).
OBD-II (On-Board Diagnostics II) is the standardized interface and protocol that lets an electronic control unit (ECU) expose diagnostic trouble codes, live engine parameters, and self-test results. The standard specifies a 16-pin connector in the cabin, a set of Mode requests (Mode 01 = live data, Mode 03 = stored DTC, Mode 09 = VIN, etc.), and a library of over 1900 standard fault codes.