On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic capability. If the vehicle's onboard diagnostic system detects a malfunction, a DTC (diagnostic trouble code) corresponding to the malfunction is stored in the vehicle's computer, and in certain cases will illuminate the MIL (malfunction indicator light, or check engine light). A service technician can retrieve the DTC, using a "scan tool", and take appropriate action to resolve the malfunction.
Prior to the advent of digital powertrain control modules which enabled the OBD feature, repairing a vehicle relied solely upon the technician's skill and service literature from the auto manufacturer.
Overview
A modern vehicle has several hundred DTC's, each of which address a particular malfunction. A simple malfunction may be that an electrical connector is disconnected, or the fuel cap is not properly installed. A DTC may indicate a component has worn out or failed. Some DTC's indicate an entire sub-system, such as the catalyst system, is not functioning properly.
The CARB (California Air Resources Board) is the leading activity for OBD regulations. The US EPA, the European Union, Japan, and other governments sometimes have unique OBD requirments as well. The SAE (Society of Automotive Engineers) standardizes the DTC's among the world's auto manufacturers. Additionally, each manufacturer can further enhance the OBD capability by adding "manufacturer specific" DTC's.
Goal
The regulatory intent of OBD is to force the auto manufacturers to design reliable emission control systems that remain effective for the vehicle's "useful life". Each vehicle's emission performance deteriorates, and the rate of deterioration varies widely depending on a number of factors such as, age, mileage, usage, geography and design. The most sophisticated features of an OBD-II system determine if the vehicle's emission performance has deteriorated below a regulated level of performance; if so, the MIL will illuminate. In California, USA for instance, renewal of a vehicle's registration will be denied if the MIL is on. In the USA (beginning in 1996), component failures that illuminate the MIL are the manufacturer's responsibility to repair, provided the vehicle is within the emission warranty period. Each manufacturer is free to employ its own engineering skill to best satisfy the regulations.
It is cost prohibitive for auto manufacturers to design an emission control system that is "deterioration proof".
OBD-II Technical Specification & Interface
OBD-II is a standardized interface to the on-board computer of a vehicle. An OBD-II interface allows for the readout of DTCs (Diagnostic Trouble Codes) that have been generated by the on-board computer, as well as realtime data from the sensors connected to the on-board computer. In addition, the OBD-II interface provides a means to clear the DTC's once maintenance has been completed.
For a list of generic OBD-II DTCs, see Table of OBD-II Codes. Individual manufactures often enhance the OBD-II code set with additional proprietary DTCs.
OBD-II Physical Layer
The OBD-II specification provides for a standarized hardware interface—the female 16-pin (2x8) J1962 connector. Unlike the OBD-I connector, which was found under the hood of the vehicle, the OBD-II connector is located on the driver's side of the passenger compartment near the center console. SAE J1962 defines the pinout of the connector as:-
The assignment of unspecified pins is left to the vehicle manufacturer's discretion.
OBD-II Signal Protocols
There are five protocols in use with the OBD-II interface, and often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:
Note that pins 4 (battery ground) and 16 (battery positive) are present in all configurations. Also, ISO 9141 and ISO 14230 use the same pinout, thus you cannot distinguish between the two simply by examining the connector.
Diagnostic data available via OBD-II
OBD-II provides access to numerous data from the ECU and offers a valuable source of information when troubleshooting problems inside a vehicle. The SAE J1979 standard defines a method for requesting various diagnostic data and a list of standard parameters that might be available from the ECU. The various parameters that are available are addressed by "parameter identification numbers" or PIDs which are defined in J1979. For a list of basic PIDs, their definitions, and the formulae to convert raw OBD-II output to meaningful diagnostic units, see OBD-II PIDs.
EOBD and JOBD
In Europe the EOBD (European On-Board Diagnostics) system was mandated by European Directive 98/69/EC for all petrol vehicles made from 1 January 2001. It is similar to the American OBD-II standard. In Japan, the JOBD system is used.
ISO15765-4 (CAN)
Some newer cars (usually post-2003) also support the CAN bus. By 2008, all vehicles sold in the US must use ISO15765-4 (a variant of CAN), which will replace all other protocols for legislated diagnostics. Vehicles sold in the United States were not allowed to use CAN for diagnostics prior to model year 2004.
OBDII Scan Tools
OBDII scan tools can be categorized in two ways, based on whether they require a computer to operate (stand-alone vs PC-based), and the intended market (professional or hobby/consumer use). Thus, all scan tools fall into one of the following four categories:
PC-Based Scan Tools
The advantages of PC-based scan tools are:
Source: Wikipedia
