OBD-II is a sort of computer which monitors emissions, mileage, speed, and other useful data. One of its strengths is it can detect problems long before the driver is able to notice any symptoms, such as low-performance, low-fuel economy, and heavy emissions, or before the Check Engine or Malfunction light comes on. Onboard Diagnostics refers to any vehicle s ability to register and report issues that may occur, or have occurred within the system. But what is it? As you would expect, OBD-II is heavily used by professional mechanics, but there are consumer oriented products that can take advantage of its diagnostic abilities. OBD-II is connected to the, which illuminates when the system detects a problem. If you purchased a car after 6996, chances are it has an OBD-II (On-board diagnostics II) port.