On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems. The amount of diagnostic information available via OBD has varied widely since the introduction in the early 1980s of on-board vehicle computers, which made OBD possible. Early instances of OBD would simply illuminate a malfunction indicator light, or MIL, 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, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.
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GM's ALDL (Assembly Line Diagnostic Link) is sometimes referred as a predecessor to, or a manufacturer's proprietary version of, an OBD-I diagnostic. 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.
http://www.webpg.net/ALDLbareINST.pdf http://www.techedge.com.au/vehicle/aldl160/160serial.htm http://tunertools.com/vendorinfo/AKM_cables/GMALDLsoftware.asp http://tunertools.com/vendorinfo/AKM_cables/AKM_electronics.asp http://www.aldlcable.com/sc/details.asp?item=aldlobd1
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, 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.
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 have an OBD and an OBD-II section in the service manual.)
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.[5]
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 normally 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 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.[6]
An OBD1.5 has also been used on Mitsubishi cars of '95 '97 vintage, some 1995 Volkswagen VR6's and in the Ford Scorpio since 95.[7]
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 engine and transmission (powertrain), B for body, C for chassis, and U for network.
The OBD-II specification provides for a standardized hardware interface—the female 16-pin (2x8) J1962 connector. 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). SAE J1962 defines the pinout of the connector as:
| 1. Manufacturer discretion. GM: J2411 GMLAN/SWC/Single-Wire CAN. | 9. - |
| 2. Bus positive Line of SAE-J1850 PWM and SAE-1850 VPW | 10. Bus negative Line of SAE-J1850 PWM only (not SAE-1850 VPW) |
| 3. Ford DCL(+) Argentina, Brazil (pre OBD-II) 1997-2000, USA, Europe, etc. Chrysler CCD Bus(+) | 11. Ford DCL(-) Argentina, Brazil (pre OBD-II) 1997-2000, USA, Europe, etc. Chrysler CCD Bus(-) |
| 4. Chassis ground | 12. - |
| 5. Signal ground | 13. - |
| 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 |
| 8. - | 16. Battery voltage |
The assignment of unspecified pins is left to the vehicle manufacturer's discretion.
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 2500kg or less) first registered within EU member states since January 1st 2001 for petrol (gasoline) engined cars and since January 1st 2004 for diesel engined cars. [8]
For newly introduced models, the regulation dates applied a year earlier - January 1st 2000 for petrol and January 1st 2003 for diesel.
For passenger cars with a Gross Vehicle Weight rating of greater than 2500kg and for light commercial vehicles, the regulation dates applied from January 1st 2002 for petrol models, and January 1st 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.
With Euro V and Euro VI emission standards, EOBD emission thresholds will be lower than previous Euro III and IV.
The term "EOBD2" is marketing speak used by some vehicle manufacturers to refer to manufacturer-specific features that are not actually part of the OBD or EOBD standard. In this case "E" stands for Enhanced.
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) standard is the Australian equivalent of OBD-II.
It applies to all vehicles of category M1 and N1 with a Gross Vehicle Weight rating of 3500kg or less, registered from new within Australia and produced since January 1st 2006 for petrol (gasoline) engined cars and since January 1st 2007 for diesel engined cars. [9]
For newly introduced models, the regulation dates applied a year earlier - January 1st 2005 for petrol and January 1st 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 3500kg or less, from July 1st 2008 for new models, July 1st 2010 for all models. [10]
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.
There are five signaling protocols that are permitted with the OBD-II interface. Most vehicles implement only one of the protocols. It is often possible to deduce the protocol used based on which pins are present on the J1962 connector:
All OBDII pinouts use the same connector but different pins are utilized with the exception of pin 4 (battery ground) and pin 16 (battery positive).
OBD-II provides access to data from the engine 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" 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:
Mode $01 is used to identify what powertrain information is available to the scan tool.
Mode $02 displays Freeze Frame data.
Mode $03 lists the emission-related "confirmed" diagnostic trouble codes stored. It displays exact numeric, 4 digit codes identifying the faults.
Mode $04 is used to clear emission-related diagnostic information. This includes clearing the stored pending/confirmed DTCs and Freeze Frame data.
Mode $05 displays the oxygen sensor monitor screen and the test results gathered about the oxygen sensor.
There are ten numbers available for diagnostics:
Mode $06 is a Request for On-Board Monitoring Test Results for Continuously and Non-Continuously Monitored System. There are typically a minimum value, a maximum value, and a current value for each non-continuous monitor.
Mode $07 is a Request for emission-related diagnostic trouble codes detected during current or last completed driving cycle. It enables the external test equipment to obtain "pending" diagnostic trouble codes detected during current or last completed driving cycle for emission-related components/systems. This is used by service technicians after a vehicle repair, and after clearing diagnostic information to see test results after a single driving cycle to determine if the repair has fixed the problem.
Mode $08 could enable the off-board test device to control the operation of an on-board system, test, or component.
Mode $09 is used to retrieve vehicle information. Among others, the following information is available:
Mode $0A lists emission-related "permanent" diagnostic trouble codes stored. As per CARB, any diagnostic trouble codes that is commanding MIL on and stored into non-volatile memory shall be logged as a permanent fault code.
Various tools are available that plug in to the OBD connector to access OBD functions. These range from simple generic consumer level tools to highly sophisticated OEM dealership tools.
A wide range of rugged hand-held scan tools is available.
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[11] or STN1110[12] 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 general 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.
In addition to proprietary software tools, free software applications are available for OBD analysis. Examples include Opendiag,[13] Freediag,[14] and pyOBD.[15]
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.
Driver's supplementary vehicle instrumentation is instrumentation 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.
Instrumentation may take the form of dedicated trip computers,[19][20] carputer or interfaces to PDAs,[21] smartphones,[22] or a Garmin navigation unit.[23]
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.