On-board diagnostics (OBD) is an automotive term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle subsystems.
The amount of diagnostic information available via OBD has varied widely since its introduction in the early 1980s versions of on-board vehicle computers. Early versions of OBD would simply illuminate a malfunction indicator light or "idiot 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 in addition to a standardized series of diagnostic trouble codes (DTCs) which allow one to rapidly identify and remedy malfunctions within the vehicle.
GM's ALDL (Assembly Line Diagnostic Link) is a General Motors proprietary onboard diagnostic interface that started with the late 1970s and early 1980s CLCC (Closed Loop Carburetor Control) and early GM EFI systems. There is an appearance of standardization because the diagnostic jack didn't change over the years ALDL was utilized by GM. GM North America used a proprietary 12 position Metripack 280 diagnostic jack. GM Australia Holden used a 6 position Metripack 280 diagnostic jack. The GM Europe Opel and Vauxhall used a 10 position Metripack 280 diagnostic jack. ALDL was not a standard. It was actually extremely fragmented. The information exchange changed with each powertrain control module (aka PCM, ECM, ECU). (A PCM integrates transmission and engine control on one Processing unit. ECM/ECU are engine control only with a separate TCM (Transmission Control Module) if needed.) While ALDL is the closest thing to standard on-board diagnostics prior to 1991 ALDL was not a standard. ALDL was even fragmented within GM brands, models, and model years. Trim levels in the same model year, division, and nameplate can use different communications. Different versions presented differences in diagnostic jack pin-outs, data protocols, and data rates (this is the reason for the ″Mask″ files needed for aftermarket software communication). Earlier versions used 160 bit/s, while later versions went up to 8192 bit/s and used bi-directional communications to the PCM or ECM/TCM.
ALDL on 1991 and later California emissions GM vehicles met the 1991 and later California OBD-I communication standard. This does not mean that ALDL is OBD-I. OBD-I was an early 1990s California-only mandate, not a United States federal mandate. It was not used on non-California emissions vehicles.
Some Asian, European, and North American diagnostic ports are sometimes incorrectly referred to as ALDL. A small number of vehicles manufactured before 1996 from other manufacturers used the GM Delphi Electronics engine and powertrain controllers; however, these used a modified ALDL communication protocol. Most did not and there was not a homogeneous name for these other proprietary diagnostic protocols and interface ports. Ford EEC, Toyota DLC, Chrysler, Nissan, Volkswagen, and others used their own on-board Diagnostics protocols and connectors, and are also not OBD-I compliant outside California.
Multiplex OBD or M-OBD is an OBD variant protocol used by Toyota, prior to OBD-II compliance. Toyota's DLC3 (Data Link Connector 3) is the standard 16-pin OBD-II connector, but a proprietary cable and software is required as generic OBD-II cables and software will not interface with it. The bus + line is SIL (Pin 7)
A 1991 and later California standard. It is not a USA Federal standard. 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 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. However, the DTCs of some OBD-I cars are interpreted in different ways. Cadillac (gasoline) fuel-injected vehicles are equipped with actual on-board 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 for OBD-II equipped vehicles.
OBD 1.5 refers to a partial implementation of OBD-II which General Motors used on some vehicles in 1994 and 1995. OBD 1.5 is a colloquialism. GM did not use the term OBD 1.5 in the documentation for these vehicles; they simply have an OBD and an OBD-II section in the service manual. Most of these 1994 & 1995 vehicles were simply 8196 baud ALDL serial data on the #9 vendor option terminal of the J1962 Jack that was formally adopted for OBD II starting in 1996.
For example, the 94–95 Corvettes have one post-catalyst oxygen sensor (although they have two catalytic converters), and have a subset of the OBD-II codes implemented. For a 1994 Corvette the implemented OBD-II codes are P0116-P0118, P0131-P0135, P0151-P0155, P0158, P0160-P0161, P0171-P0175, P0420, P1114-P1115, P1133, P1153 and P1158.
This hybrid system was present on the GM H-body cars in 94–95, W-body cars (Buick Regal, Chevrolet Lumina ('95 only), Chevrolet Monte Carlo ('95 only), Pontiac Grand Prix, Oldsmobile Cutlass Supreme) in 94–95, L-body (Chevrolet Beretta/Corsica) in 94–95, Y-body (Chevrolet Corvette) in 94–95, on the F-body (Chevrolet Camaro and Pontiac Firebird) in 95 and on the J-Body (Chevrolet Cavalier and Pontiac Sunfire) and N-Body (Buick Skylark, Oldsmobile Achieva, Pontiac Grand Am) in 95 and 96 and also on '94–'95 Saab vehicles with the naturally aspirated 2.3.
The pinout for the ALDL connection on these cars is as follows:
GM used at least two (#9 & #12) of what became seven "Vendor Option" terminals (1, 8, 9, 11, 12, 13) along with #4 Chassis Ground and #16 Battery Power in the formally accepted J1962 Jack. While OBD II interfaces will not communicate with these controllers they will not be damaged by plugging into these jacks either.
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.
Codes retrieved are still 2 digit codes which still require an ALDL scan tool, a laptop and USB-ALDL interface with a properly pinned J1962 ALDL plug, or a GM Tech II. Flash codes can be retrieved on 1994–1995 Corvettes by shorting #12-Vendor Option to #4 Chassis Ground.
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 a list of standardized DTCs. As a result of this standardization, a single device can query the on-board computer(s) for these parameters in any vehicle. OBD-II standardization was prompted to simplify diagnosis of increasingly complicated emissions equipment, and though only emission-related codes and data are required to be transmitted through it according to U.S. legislation, most manufacturers have made the OBD-II Data Link Connector the main connector in the vehicle through which all systems are diagnosed and reprogrammed. OBD-II Diagnostic Trouble Codes are 4-digit, preceded by a letter: P for engine and transmission (powertrain), B for body, C for chassis, and U for network. Manufacturers may also add custom data parameters to their specific OBD-II implementation, including real-time data requests as well as trouble codes.
The SAE J1962 specification provides for two standardized hardware interfaces, called type A and type B. Both are female, 16-pin (2x8), D-shaped connectors, and both have a groove between the two rows of pins, but type B's groove is interrupted in the middle. This prevents the insertion of a type A male plug into a type B female socket while allowing a type B male plug to be inserted into a type A female socket.
The type A connector is used for vehicles that use 12V supply voltage, whereas type B is used for 24V vehicles and it is required to mark the front of the D-shaped area in blue color.
SAE J1962 defines the pinout of the connector as:
|1||Manufacturer discretion:||9||Manufacturer discretion:
|2||Bus Positive Line of SAE J1850 PWM and VPW||10||Bus Negative Line of SAE J1850 PWM only (not SAE J1850 VPW)|
|3||Manufacturer Discretion:||11||Manufacturer Discretion:|
|4||Chassis ground||12||Manufacturer discretion:
|5||Signal ground||13||Manufacturer discretion:
|6||CAN-High (ISO 15765-4 and SAE J2284)||14||CAN-Low (ISO 15765-4 and SAE J2284)|
|7||K-Line of ISO 9141-2 and ISO 14230-4||15||L-Line of ISO 9141-2 and ISO 14230-4|
Unlike the OBD-I connector, which was sometimes found under the hood of the vehicle, the OBD-II connector is required to be within 2 feet (0.61 m) 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).
The EOBD (European on board diagnostics) regulations are the European equivalent of OBD-II, and apply to all passenger cars of category M1 (with no more than 8 passenger seats and a Gross Vehicle Weight rating of 2500 kg or less) first registered within EU member states since 1 January 2001 for petrol (gasoline) engined cars and since 1 January 2004 for diesel engined cars.
For newly introduced models, the regulation dates applied a year earlier – 1 January 2000 for petrol and 1 January 2003 for diesel. For passenger cars with a Gross Vehicle Weight rating of greater than 2500 kg and for light commercial vehicles, the regulation dates applied from 1 January 2002 for petrol models, and 1 January 2007 for diesel models.
The technical implementation of EOBD is essentially the same as OBD-II, with the same SAE J1962 diagnostic link connector and signal protocols being used.
In 2017, all previous standards were revoked because there were more than 24 standards produced over 35 years. The new document supplanted all previous versions.
Each of the EOBD fault codes consists of five characters: a letter, followed by four numbers. The letter refers to the system being interrogated e.g. Pxxxx would refer to the powertrain system. The next character would be a 0 if complies to the EOBD standard. So it should look like P0xxx.
The first letter indicates the family of DTC.
The first digit indicates if the code is generic or not (green digit):
The next character would refer to the sub system.
The last two characters ("xx") refer to the individual fault within each subsystem.
JOBD is a version of OBD-II for vehicles sold in Japan.
The ADR 79/01 vehicle standard (Australian Design Rule 79/01 – Emission Control for Light Vehicles, 2005) is the Australian equivalent of OBD-II. It applies to all vehicles of category M1 and N1 with a gross Vehicle Weight rating of 3500 kg or less, registered from new within Australia and produced since 1 January 2006 for petrol (gasoline) engined cars and since 1 January 2007 for diesel engined cars. For newly introduced models, the regulation dates applied a year earlier – 1 January 2005 for petrol and January 2006 for diesel. The ADR 79/01 standard was supplemented by the ADR 79/02 standard which imposed tighter emissions restrictions, applicable to all vehicles of class M1 and N1 with a gross vehicle weight rating of 3500 kg or less, from 1 July 2008 for new models, 1 July 2010 for all models. The technical implementation of this standard is essentially the same as OBD-II, with the same SAE J1962 diagnostic link connector and signal protocols being used.
Five signaling protocols are permitted with the OBD-II interface; most vehicles implement only one. It is often possible to deduce the protocol, based on which pins are present on the J1962 connector:
All OBD-II pinouts use the same connector, but different pins are used with the exception of pin 4 (battery ground) and pin 16 (battery positive).
OBD-II provides access to data from the electronic control unit (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" (parameter IDs or PIDs) which are defined in J1979. For a list of basic PIDs, their definitions, and the formula to convert raw OBD-II output to meaningful diagnostic units, see OBD-II PIDs. Manufacturers are not required to implement all PIDs listed in J1979 and they are allowed to include proprietary PIDs that are not listed. The PID request and data retrieval system gives access to real time performance data as well as flagged DTCs. For a list of generic OBD-II DTCs suggested by the SAE, see Table of OBD-II Codes. Individual manufacturers often enhance the OBD-II code set with additional proprietary DTCs.
Here is a basic introduction to the OBD communication protocol according to ISO 15031-5:
Various tools are available that plug into the OBD connector to access OBD functions. These range from simple generic consumer level tools to highly sophisticated OEM dealership tools to vehicle telematic devices.
A range of rugged hand-held scan tools is available.
Mobile device applications allow mobile devices such as cell phones and tablets to display and manipulate the OBD-II data accessed via USB adaptor cables, bluetooth or WiFi adapters plugged into the car's OBD-II connector. A number of new devices allow the vehicle's OBD-II port to stream data directly to the Internet via a cellular connection.
A PC-based OBD analysis tool that converts the OBD-II signals to serial data (USB or serial port) standard to PCs or Macs. The software then decodes the received data to a visual display. Many popular interfaces are based on the ELM or STN OBD Interpreter ICs, both of which read all five generic OBD-II protocols. Some adapters now use the J2534 API allowing them to access OBD-II Protocols for both cars and trucks.
In addition to the functions of a hand-held scan tool, the PC-based tools generally offer:
The extent that a PC tool may access manufacturer or vehicle-specific ECU diagnostics varies between software products as it does between hand-held scanners.
Data loggers are designed to capture vehicle data while the vehicle is in normal operation, for later analysis.
Data logging uses include:
Analysis of vehicle black box data may be performed on a periodic basis, automatically transmitted wirelessly to a third party or retrieved for forensic analysis after an event such as an accident, traffic infringement or mechanical fault.
In the United States, many states now use OBD-II testing instead of tailpipe testing in OBD-II compliant vehicles (1996 and newer). Since OBD-II stores trouble codes for emissions equipment, the testing computer can query the vehicle's onboard computer and verify there are no emission related trouble codes and that the vehicle is in compliance with emission standards for the model year it was manufactured.
In the Netherlands, 2006 and later vehicles get a yearly EOBD emission check.
Driver's supplementary vehicle instrumentation is installed in a vehicle in addition to that provided by the vehicle manufacturer and intended for display to the driver during normal operation. This is opposed to scanners used primarily for active fault diagnosis, tuning, or hidden data logging.
Auto enthusiasts have traditionally installed additional gauges such as manifold vacuum, battery current etc. The OBD standard interface has enabled a new generation of enthusiast instrumentation accessing the full range of vehicle data used for diagnostics, and derived data such as instantaneous fuel economy.
As a carputer is essentially a PC, the same software could be loaded as for PC-based scan tools and vice versa, so the distinction is only in the reason for use of the software.
These enthusiast systems may also include some functionality similar to the other scan tools.
OBD II is no longer only used by professionals and hobbyists to repair vehicles. OBD II information is commonly used by vehicle telematics devices that perform fleet tracking, monitor fuel efficiency, prevent unsafe driving, as well as for remote diagnostics and by pay-as-you-drive insurance. Although originally not intended for the above purposes, commonly supported OBD II data such as vehicle speed, RPM, and fuel level allow GPS-based fleet tracking devices to monitor vehicle idling times, speeding, and over-revving. By monitoring OBD II DTCs a company can know immediately if one of its vehicles has an engine problem and by interpreting the code the nature of the problem. OBD II is also monitored to block mobile phones when driving and to record trip data for insurance purposes.
Since 2010, Title 13 of the California Code of Regulations 1971.1 allows heavy duty (class 8) diesel trucks to use either SAE J1939-73 OBD protocols with the J1939-13 round OBD connector, or SAE J1979 OBD protocols with the J1962 OBD connector (same as passenger cars). Heavy trucks that use J1979/J1962 (for example Mack and Volvo Trucks North America) typically use 29 bit CAN identifiers.
Researchers at the University of Washington and University of California examined the security around OBD, and found that they were able to gain control over many vehicle components via the interface. Furthermore, they were able to upload new firmware into the engine control units. Their conclusion is that vehicle embedded systems are not designed with security in mind.
There have been reports of thieves using specialist OBD reprogramming devices to enable them to steal cars without the use of a key. The primary causes of this vulnerability lie in the tendency for vehicle manufacturers to extend the bus for purposes other than those for which it was designed, and the lack of authentication and authorization in the OBD specifications, which instead rely largely on security through obscurity. The National Highway Traffic Safety Administration has demonstrated the ability to take over certain functions through wires to the car's control center.
In 2012, vehicles produced by BMW, Porsche, Opel, Renault, Mercedes, Volkswagen and Toyota were stolen by programming a blank key fob to start the car through the OBD connection. BMW offered all owners a free fix through a software update, and all newer vehicles have upgraded software that fixed this vulnerability.
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