On-board diagnostics (OBD) is a term referring to a vehicle's self-diagnostic and reporting capability. In the United States, this capability is a requirement to comply with federal emissions standards to detect failures that may increase the vehicle tailpipe emissions to more than 150% of the standard to which it was originally certified.
OBD systems give the vehicle owner or repair technician access to the status of the various vehicle sub-systems. The amount of diagnostic information available via OBD has varied widely since its introduction in the early 1980s versions of onboard vehicle computers. Early versions of OBD would simply illuminate a tell-tale light if a problem was detected, but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized digital communications port to provide real-time data and diagnostic trouble codes which allow malfunctions within the vehicle to be rapidly identified.
History
1968: Volkswagen introduces the first on-board computer system, in their fuel-injected Type 3 models. This system is entirely analog with no diagnostic capabilities. 1975: Bosch and Bendix EFI systems are adopted by major automotive manufacturers to improve tailpipe (exhaust) emissions. These systems are also analog, though some provide rudimentary diagnostic capability through factory tools, such as the Kent Moore J-25400, compatible with the Datsun 280Z, and the Cadillac Seville. 1980: General Motors introduces the first data link on their 1980 Cadillac Eldorado and Seville models. Diagnostic Trouble Codes (DTCs) are displayed through the electronic climate control system's digital readout when in diagnostic mode. 1981: General Motors introduced its "Computer Command Control" system on all US passenger vehicles for model year 1981. Included in this system is a proprietary 5-pin ALDL that interfaces with the Engine Control Module (ECM) to initiate a diagnostic request and provide a serial data stream. The protocol communicates at 160 baud with Pulse-width modulation (PWM) signaling and monitors all engine management functions. It reports real-time sensor data, component overrides, and Diagnostic Trouble Codes. The specification for this link is as defined by GM's Emissions Control System Project Center document XDE-5024B. 1982: RCA defines an analog STE/ICE (simplified test equipment for internal combustion engines) vehicle diagnostic standard used in the CUCV, M60 tank and other military vehicles of the era for the US Army. 1986: General Motors introduces an upgraded version of the ALDL protocol, which communicates at 8192 baud with half-duplex UART signaling on some models. 1988: The California Air Resources Board (CARB) requires that all new vehicles sold in California from 1988 onward have some basic OBD capability (such as detecting problems with fuel metering and exhaust gas recirculation.) These requirements are generally referred to as "OBD-I", though this name is a retronym applied after the introduction of OBD-II. The data link connector and its position are not standardized, nor is the data protocol. The Society of Automotive Engineers (SAE) recommends a standardized diagnostic connector and set of diagnostic test signals. ~1994: Motivated by a desire for a state-wide emissions testing program, the CARB issues the OBD-II specification and mandates that it be adopted for all cars sold in California starting in model year 1996 (see CCR Title 13 Section 1968.1 and 40 CFR Part 86 Section 86.094). The DTCs and connectors suggested by the SAE are incorporated into this specification. 1996: The OBD-II specification is made mandatory for all passenger cars and petrol-powered light trucks with a gross vehicle weight rating less than in the United States. The OBD-II specification is also made mandatory for all petrol-powered vehicles with California emissions with a gross vehicle weight rating up to . 1997: The OBD-II specification is made mandatory for California emissions diesel-engined vehicles with a gross vehicle weight rating up to . 2001: The European Union makes EOBD mandatory for all petrol vehicles sold in the European Union, starting in MY2001 (see European emission standards Directive 98/69/EC). 2004: The European Union makes EOBD mandatory for all diesel vehicles sold in the European Union. All petrol-powered vehicles in the United States with a gross vehicle weight rating of up to are required to have OBD-II. 2006: All vehicles manufactured in Australia and New Zealand are required to be OBD-II compliant after January 1, 2006. All vehicles in the United States of gross vehicle weight rating and under are required to have OBD-II. 2007: All California emissions vehicles over gross vehicle weight rating are required to support EMD/EMD+ or OBD-II. 2008: All cars sold in the United States are required to use the ISO 15765-4 signaling standard (a variant of the Controller Area Network (CAN) bus). 2008: Certain light vehicles in China are required by the Environmental Protection Administration Office to implement OBD (standard GB18352) by July 1, 2008. Some regional exemptions may apply. 2010: Required phase-in of the OBD-II specification to all vehicles with a gross vehicle weight rating of and above was initiated in the United States. This was completed by the 2013 model year. Vehicles that did not have OBD-II during this time period were required to have EMD/EMD+.
Standard interfaces
ALDL
GM's ALDL (Assembly Line Diagnostic Link) is sometimes referred to as a predecessor to, or a manufacturer's proprietary version of, an OBD-I diagnostic starting in 1981. This interface was made in different varieties and changed with power train control modules (aka PCM, ECM, ECU). Different versions had slight differences in pin-outs and baud rates. Earlier versions used a 160 baud rate, while later versions went up to 8192 baud and used bi-directional communications to the PCM.
OBD-I
The regulatory intent of OBD-I was to encourage auto manufacturers to design reliable emission control systems that remain effective for the vehicle's "useful life". The hope was that by forcing annual emissions testing for California starting in 1988, and denying registration to vehicles that did not pass, drivers would tend to purchase vehicles that would more reliably pass the test. OBD-I was largely unsuccessful, as the means of reporting emissions-specific diagnostic information was not standardized. Technical difficulties with obtaining standardized and reliable emissions information from all vehicles led to an inability to implement the annual testing program effectively.
The Diagnostic Trouble Codes (DTCs) of OBD-I vehicles can usually be found without an expensive scan tool. Each manufacturer used their own Diagnostic Link Connector (DLC), DLC location, DTC definitions, and procedure to read the DTCs from the vehicle. DTCs from OBD-I cars are often read through the blinking patterns of the 'Check Engine Light' (CEL) or 'Service Engine Soon' (SES) light. By connecting certain pins of the diagnostic connector, the 'Check Engine' light will blink out a two-digit number that corresponds to a specific error condition. The DTCs of some OBD-I cars are interpreted in different ways, however. Cadillac fuel-injected vehicles are equipped with actual onboard diagnostics, providing trouble codes, actuator tests and sensor data through the new digital Electronic Climate Control display.
Holding down 'Off' and 'Warmer' for several seconds activates the diagnostic mode without the need for an external scan tool. Some Honda engine computers are equipped with LEDs that light up in a specific pattern to indicate the DTC. General Motors, some 1989–1995 Ford vehicles (DCL), and some 1989–1995 Toyota/Lexus vehicles have a live sensor data stream available; however, many other OBD-I equipped vehicles do not. OBD-I vehicles have fewer DTCs available than OBD-II equipped vehicles.
OBD-1.5
OBD 1.5 refers to a partial implementation of OBD-II which General Motors used on some vehicles in 1994, 1995 & 1996 (GM did not use the term OBD 1.5 in the documentation for these vehicles — they simply had an OBD and an OBD-II section in the service manual).
For example, the 1994–1995 model year Corvettes have one post-catalyst oxygen sensor (although they have two catalytic converters), and have a subset of the OBD-II codes implemented.
This hybrid system was present on GM B-body cars (the Chevrolet Caprice, Impala, and Buick Roadmaster) for 1994–1995 model years, H-body cars for 1994–1995, W-body cars (Buick Regal, Chevrolet Lumina) for 1995 only, Chevrolet Monte Carlo (1995 only), Pontiac Grand Prix, Oldsmobile Cutlass Supreme (for 1994–1995), L-body (Chevrolet Beretta/Corsica) for 1994–1995, Y-body (Chevrolet Corvette) for 1994–1995, on the F-body (Chevrolet Camaro and Pontiac Firebird) for 1995 and on the J-Body (Chevrolet Cavalier and Pontiac Sunfire) and N-Body (Buick Skylark, Oldsmobile Achieva, Pontiac Grand Am) for 1995 and 1996 and also for North American delivered 1994–1995 Saab vehicles with the naturally aspirated 2.3.
The pinout for the ALDL connection on these cars is as follows: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
For ALDL connections, pin 9 is the data stream, pins 4 and 5 are ground, and pin 16 is the battery voltage.
An OBD 1.5 compatible scan tool is required to read codes generated by OBD 1.5.
Additional vehicle-specific diagnostic and control circuits are also available on this connector. For instance, on the Corvette there are interfaces for the Class 2 serial data stream from the PCM, the CCM diagnostic terminal, the radio data stream, the airbag system, the selective ride control system, the low tire pressure warning system, and the passive keyless entry system.
An OBD 1.5 has also been used in the Ford Scorpio since 95.
OBD-II OBD-II is an improvement over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signalling protocols available, and the messaging format. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for each. There is a pin in the connector that provides power for the scan tool from the vehicle battery, which eliminates the need to connect a scan tool to a power source separately. However, some technicians might still connect the scan tool to an auxiliary power source to protect data in the unusual event that a vehicle experiences a loss of electrical power due to a malfunction. Finally, the OBD-II standard provides an extensible list of DTCs. As a result of this standardization, a single device can query the on-board computer(s) in any vehicle. This OBD-II came in two models OBD-IIA and OBD-IIB. OBD-II standardization was prompted by emissions requirements, and though only emission-related codes and data are required to be transmitted through it, most manufacturers have made the OBD-II Data Link Connector the only one in the vehicle through which all systems are diagnosed and programmed. OBD-II Diagnostic Trouble Codes are 4-digit, preceded by a letter: P for powertrain (engine and transmission), B for body, C for chassis, and U for network.
OBD-II diagnostic connector
The OBD-II specification provides for a standardized hardware interface — the female 16-pin (2x8) J1962 connector, where type A is used for 12-volt vehicles and type B for 24-volt vehicles. Unlike the OBD-I connector, which was sometimes found under the bonnet of the vehicle, the OBD-II connector is required to be within of the steering wheel (unless an exemption is applied for by the manufacturer, in which case it is still somewhere within reach of the driver).
SAE J1962 defines the pinout of the connector as:
1 Manufacturer discretion GM: J2411 GMLAN/SWC/Single-Wire CAN. Audi: Switched +12 to tell a scan tool whether the ignition is on.