What is an OBD (on-board diagnostics)?

Introduction to the OBD II Standard

On-board diagnostics (OBD) is a computer-based self-diagnostic, monitoring, and reporting system built into automobiles to monitor the performance of the various engine components and emission system components.

It is used to implement vehicle diagnostics communication for diagnosis and repair of vehicle sub-systems through communication with Electronic Control Units (ECU). ECUs monitor and control the sub-systems of a vehicle. Common ECUs include Engine Control Module (ECM), Transmission Control Module (TCM), Electronic Brake Control Module (EBCM), etc. Thus OBD helps to detect and control engine failures, performance issues and fight vehicle emission.
Emission Control Regulations and OBD Requirements

The US federal emission standards for engines and vehicles are established by the US Environmental Protection Agency (EPA). The EPA emission standards for regulating engine emissions and air quality in general, is based on the Clean Air Act (1970), which is most recently amended in 1990. California is the only state that has the authority to adopt its own emission regulations. Other states have the option to either implement the federal emission standards or adopt California requirements (CAA Section 177).

During the early 1980’s, manufacturers started using electronic monitoring systems to control engine functions, fuel feed, ignition systems and emission systems, diagnose problems and alert users. This is called Engine/Electronic Control Unit (ECU) or Module (ECM).

OBD is the language of ECU, and OBD regulations ensure compliance with Californian and federal emission standards. This is achieved by setting OBD requirements to diagnose and monitor the vehicle emission system components such as catalytic convertors and to alert the driver/operator upon detecting any problems.
From 1994 onwards, EPA regulations require presence of OBD systems in light duty vehicles (LDV) and light duty trucks (LDT). OBD-II, an enhanced capability OBD standard computer system is mandatory for vehicle models from 1996 onwards. Since 2005, OBD has become mandatory for heavy duty vehicles (HDV) also.
The Evolution of OBD

OBD systems provide access to the status of various vehicle sub systems. This information can be utilized by the vehicle owner, driver, repair technician or testing authority to test and evaluate the condition and performance of the vehicle. The feedback provided by early OBD systems (1980s) were simple illumination or malfunction indicator lights on the vehicle dashboard. It was not sufficient for identifying the nature of the problem.

OBD has evolved since that and now the latest standard named OBD-II (1994), the successor of OBD-I and OBD-1.5, uses a standard digital communications port to provide real-time vehicle data along with a standardized series of Diagnostic Trouble Codes (DTC). This allows the user to identify the nature of the problem and enables him to correct the malfunctions of the vehicle.

OBD-II not only controls engine functions, but also monitors other vehicle components like chassis, body, accessory devices like mirrors, wipers etc and also the diagnostic control network of the vehicle. It also follows the California Air Resources Board (CARB) guidelines for data storage and accessibility to be provided to the external scan tools. Thus OBD-II is a protocol for vehicle diagnostics communication.
The OBD-II Standard

The ISO 15031 standard specifies a protocol for communication between a vehicle (through the OBD port/connector) and en external diagnostic equipment for emissions related diagnostics. The OBD-II standard is a set of specifications and services based on the ISO 15031 standard and specify the type of OBD diagnostic connector, the electrical signaling protocols and the messaging format. It also specifies a list of vehicle parameters to be monitored and also information about how to encode the data associated with these parameters during transmission and storage.

From the OBD port fitted in the vehicle and through the OBD diagnostic connector, we can access vehicle parameters like speed, engine data, emission data etc. in the form of DTCs. The OBD-II standard provides a list of standardized DTCs. The OBD-II standards have prompted design of standard devices that can query for particular information like value of a vehicle parameter. This query, the diagnostic request is sent to the concerned ECU in the vehicle. The ECU responds with applicable DTCs, termed as the diagnostic response.

For this diagnostic communication, the physical medium or signaling protocol will be like the CAN (Controller Area Network) automotive protocol. The OBD-II standardization has simplified diagnosis of emission related vehicle sub-systems by the usage of emission-related codes and data through the CAN bus.

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