What Is OBD-II? Definition, Meaning and Concept

OBD-II is a system used in automobiles to monitor various vehicle components, detect faults, and store the information in the vehicle's on-board computer for later retrieval by a service technician. OBD-II is an acronym for On-Board Diagnostics; the "II" denotes the second and most current version of this technology. Beginning in the early 1970s, vehicles sold in the United States have been equipped with electronic devices to monitor various systems and diagnose malfunctions with the goal of minimizing pollution. This occurred in response to Congress passing the Clean Air Act and establishing the Environmental Protection Agency (EPA) in 1970. These electronic components varied between manufacturers and model years,

In 1988, the EPA and the California Air Resources Board (CARB) mandated that vehicle manufacturers include self-test programs to ensure their emissions equipment would remain effective for the life of the vehicle. The Society of Automotive Engineers standardized a connector and set of diagnostic test signals. Upon equipment failure, this system illuminated a malfunction indicator light (MIL) on the vehicle's dash, often called the "check engine" light. Required on all 1991 and newer cars, this system became known as On-Board Diagnostics I, or OBD-I.

CARB studies soon concluded that OBD-I systems would not detect emissions components unless they failed completely, and in some cases the vehicle could still pass an emissions test. The new laws and requirements became effective on January 1, 1996, the standard adopted for OBD-II. All vehicles built for sale in the US on or after that date are OBD-II equipped.

OBD-II uses various sensors throughout the car to provide the computer, also called the electronic control module (ECM), with information such as engine and ambient temperatures, vehicle speed, etc. The ECM then advances or retards the ignition timing and adds or subtracts fuel accordingly. It also tests the signals from all connected sensors. When a signal is missing or out of spec, the OBD-II system illuminates the MIL and stores its corresponding diagnostic trouble code in its memory.

OBD-II memory information is read through a connector inside the car. OBD-II improves on OBD-I not only in its more sophisticated diagnostic capabilities, but also because it allows three types of data to be read: trouble codes, real-time data - the raw sensor information reported to the OBD-computer. II and Freeze Frame Data: A "snapshot" of the sensor data at the time the car's MIL was turned on. OBD-II codes are read using cables and software created to communicate with OBD-II systems. These can take the form of stand-alone drives or software that is installed on a PC. Some are complex models intended for professional technicians; The simpler units are priced for hobbyists.