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on-board diagnostics (OBD)

Contributor(s): Matthew Haughn

On board-diagnostics (OBD) is the inclusion of circuitry within a device that can indicate proper function and calibration of the main unit or specific fault states.

ODB has been used in cars since the 1960s. Most modern cars feature OBD, generally using computers to interpret issues. The systems became a near-necessity with the introduction of fuel injection, where they enable tuning for the more advanced method of mixing fuel and air for combustion. There have been different standards and connectors for OBD in cars but most now use a standardized connection OBD-II and codes made mandatory for any cars sold in the United States since 1996.

In an ODB, an electronic control unit (ECU) takes input from various sensors and uses that information to control actuators. On-board diagnostics can take the form of hand-held multi-standard OBD computers with connectors and software for PC hook-up. These connection methods make it possible for mechanics to perform diagnostics and adjust ECUs for better fuel economy or performance. OBD is also used in emission testing, querying the exhaust system ECU for problems. In fleet management systems, on-board diagnostics can monitor and detect issues involving fuel efficiency, GPS and even unsafe driving.

Issues in electronics may be indicated in different ways by OBD. During initialization, a diagnostic device may flip through a number of codes if a process stops unexpectedly during startup. The user can look up the code that remains after the stop. Another common system presents a two-character readout. The codes generally require documentation to interpret the indicated problem. An older, simpler form of OBD just lit indicator lights associated with alerts like, for example, "check engine.” Some OBD systems require a computer for interpretation, and some can deliver speech-based alerts.

As more devices use electronic controls, OBD are increasingly important for finding issues because it's not possible to physically observe the workings of non-mechanical parts. However, although electronic systems are widely used, in many cases mechanical parts can do as good a job and offer greater serviceability for end users.

This was last updated in October 2017

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