On Board Diagnostics

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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 were detected—but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized fast digital communications port to provide myriad realtime data in addition to a standardized series of Table of OBD-II Codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.

History

1970: The United States Congress passes the Clean Air Act and establishes the United States Environmental Protection Agency.
~1980: On-board computers begin appearing on consumer vehicles, largely motivated by their need for real-time tuning of fuel injection systems. Simple OBD implementations appear, though there is no standardization in what is monitored or how it is reported.
1982: General Motors implements an internal standard for its OBD called the Assembly Line Communications Link (ALCL), later renamed the Assembly Line Diagnostics Link (ALDL). The initial ALCL protocol communicates at 160 baud with Pulse-width modulation (PWM) signaling and monitors very few vehicle systems.
1986: An upgraded version of the ALDL protocol appears which communicates at 8192 baud with half-duplex UART signaling. This protocol is defined in GM XDE-5024B.
~1987: The California Air Resources Board (CARB) requires that all new vehicles sold in California starting in manufacturer's year 1988 (MY1988) have some basic OBD capability. The requirements they specify are generally referred to as the "OBD-I" standard, though this name is not applied until the introduction of OBD-II. The data link connector and its position are not standardized, nor is the data protocol.
1988: The Society of Automotive Engineers (Society of Automotive Engineers) recommends a standardized diagnostic connector and set of diagnostic test signals.
~1994: Motivated by a desire for a state-wide Automobile emissions control 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 connector suggested by the Society of Automotive Engineers are incorporated into this specification.
1996: The OBD-II specification is made mandatory for all cars sold in the United States.
2001: The European Union makes EOBD , a variant of OBD-II, mandatory for all petrol vehicles sold in the European Union, starting in MY2001 (see European emission standards Directive 98/69/EC ).
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 (computing)).

Standard interfaces ALDL/ALCL The Assembly Line Communications Link (ALCL) was later renamed the Assembly Line Diagnostic Link (ALDL). The two terms are used synonymously. This system was only vaguely standardized and suffered from the fact that specifications for the communications link varied from one model to the next. ALDL was largely used by manufacturers for diagnostics at their dealerships and official maintenance facilities.

Diagnostic connector There were at least three different connectors used with ALDL. General Motors implemented both a 5-pin connector and a 12-pin connector, with the 12 pin connector being used in the vast majority of GM cars. Lotus implemented a 10-pin connector. The pins are given letter designations in the following layouts (as seen from the front of the vehicle connector):

12-pin ALDL connector pinout:
F E D C B A
G H J K L M

10-pin ALDL connector pinout:
A B C D E
K J H G F

5-pin ALDL connector pinout:
A B C D E

Note the difference in pin ordering between the connectors and the fact that the letter I is not used. Unfortunately, the definition of which signals were present on each pin varied between vehicle models. There were generally only three pins used for basic ALDL —ground, battery voltage, and a single line for data—, although other pins were often used for additional vehicle-specific diagnostic information and control interfaces. No battery voltage is present in the 12 pin ALDL connector.

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, and denying registration to vehicles that did not pass, drivers would tend to purchase vehicles that would more reliably pass the test. Along these lines, OBD-I was largely unsuccessful—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 effectively implement the annual testing program.

OBD 1.5 "OBD 1.5" is a slang term referring to a partial implementation of OBD-II which GM used on some vehicles in 1994 and 1995 (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.) This hybrid system was present on the H-body cars in 94-95, L-body (Chevrolet Beretta / Chevrolet 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.

Depending on the year and the vehicle, a car with the OBD 1.5 system may have either the older OBD-I connector, or the newer OBD-II connector, but they are electrically identical to each other. For example, the 94-95 Corvettes have one post-cat 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.

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.

Additional vehicle-specific diagnostic and control circuits are 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 OBD1.5 has also been used on Mitsubishi cars of '95 '97 vintage.

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. 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 simplification of reporting diagnostic data led the feasibility of the comprehensive emissions testing program envisioned by the CARB.

OBD-II Diagnostic connector 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 nearly always located on the driver's side of the passenger compartment near the center console. Society of Automotive Engineers J1962 defines the pinout of the connector as:

Bus positive Line of SAE-J1850

Chassis ground

Signal ground

CAN high (ISO 15765-4 and SAE-J2234)

K line of ISO 9141-2 and ISO 14230-4

Bus negative Line of Society of Automotive Engineers-J1850

CAN low (ISO 15765-4 and Society of Automotive Engineers-J2234)

L line of ISO 9141-2 and ISO 14230-4

Battery voltage

The assignment of unspecified pins is left to the vehicle manufacturer's discretion.

Signal protocols There are five signalling protocols currently in use with the OBD-II interface. Any given vehicle will likely only implement one of the protocols. Often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:

Society of Automotive Engineers J1850 PWM (pulse-width modulation - 41.6 kbaud, standard of the Ford Motor Company)

pin 2: Bus+
pin 10: Bus–
High voltage is +5 V
Message length is restricted to 11 bytes, including Cyclic redundancy check
Employs a multi-master arbitration scheme called 'Carrier Sense Multiple Access with Non-Destructive Arbitration' (CSMA/NDA)

SAE J1850 VPW (variable pulse width - 10.4/41.6 kbaud, standard of General Motors Corporation)

pin 2: Bus+
Bus idles low
High voltage is +7 V
Decision point is +3.5 V
Message length is restricted to 11 bytes, including CRC
Employs Carrier Sense Multiple Access/NDA

International Organization for Standardization 9141-2. This protocol has a data rate of 10.4 kbaud, and is similar to RS-232. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.

pin 7: K-line
pin 15: L-line (optional)
UART signaling (though not RS-232 voltage levels)
K-line idles high
High voltage is Vbatt
Message length is restricted to 11 bytes, including CRC

ISO 14230 KWP2000 (Keyword Protocol 2000)

pin 7: K-line
pin 15: L-line (optional)
Physical layer identical to ISO 9141-2
Data rate 1.2 to 10.4 kbaud
Message may contain up to 255 bytes in the data field

ISO 15765 Controller Area Network (250 kbit/s or 500 kbit/s). The CAN protocol is a popular standard outside of the US automotive industry and is making significant in-roads into the OBD-II market share. By 2008, all vehicles sold in the US will be required to implement CAN, thus eliminating the ambiguity of the existing five signalling protocols.

pin 6: CAN High
pin 14: CAN Low

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 the connector shape does not distinguish between the two.

Diagnostic data available OBD-II provides access to numerous data from the ECU (Electronic Control Unit) and offers a valuable source of information when troubleshooting problems inside a vehicle. The Society of Automotive Engineers 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. 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 manufactures often enhance the OBD-II code set with additional proprietary DTCs.

Scan tools OBD scan tools can be categorized in several ways ranging from whether they are O.E.M. tools or aftermarket tools, whether they require a computer to operate (stand-alone tool vs PC-based software), and the intended market (professional or hobby/consumer use).

The advantages of PC-based scan tools are:

Low cost (compared to stand-alone scan tools with similar functionality -if you don't count the cost of a laptop PC).
Virtually unlimited storage capacity for data logging and other functions.
Higher resolution screen than handheld tools.
Availability of multiple software programs.
Some are capable of reprogramming

The advantages of stand-alone tools:

Wide selection beginning with simple code read/erase tools starting at as low as $79 retail.
Simplified operation that requires no computer skills/ PC compatibility issues.
Rugged designs, intended for use in and around cars (i.e. no lugging a $1500 laptop in and around a car).

Mode of Operation 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 total number of powertrain or emission related DTC stored. It also displays exact numeric, 5 digit codes identifying the faults.

Mode $04 is used to clear DTCs and Freeze Frame.

Mode $05 displays the oxygen sensor monitor screen and the test results gathered about the oxygen sensor.

There are ten numbers available for diagnostics:

$01 Rich-to-Lean O2 sensor threshold voltage

$02 Lean-to-Rich O2 sensor threshold voltage

$03 Low sensor voltage threshold for switch time measurement

$04 High sensor voltage threshold for switch time measurement

$05 Rich-to-Lean switch time in ms

$06 Lean-to Rich switch time in ms

$07 Minimum voltage for test

$08 Maximum voltage for test

$09 Time between voltage transitions in ms

Mode $06 is a Request for On-Board Monitoring Test Results for 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 continuously Monitored 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.

There are only three continuous monitors to be identified: fuel, misfire, and the comprehensive component.

Mode $08 could enable the off-board test device to control the operation of an on-board system, test, or component.

Standards documents Society of Automotive Engineers standards documents on OBD-II

J1962 - Defines the physical connector used for the OBD-II interface.
J1850 - Defines a serial data protocol. There are 2 variants- 10.4 kbit/s (single wire, VPW) and 41.6 kbit/s (2 wire, PWM). Mainly used by US manufacturers, also known as PCI (Chrysler, 10.4K), Class 2 (GM, 10.4K), and SCP (Ford, 41.6K)
J1978 - Defines minimal operating standards for OBD-II scan tools
J1979 - Defines standards for diagnostic test modes
J2012 - Defines standards trouble codes and definitions.
J2178-1 - Defines standards for network message header formats and physical address assignments
J2178-2 - Gives data parameter definitions
J2178-3 - Defines standards for network message frame IDs for single byte headers
J2178-4 - Defines standards for network messages with three byte headers*
J2284-3 - Defines 500K Controller area network Physical layer and Data link layer Link Layer

ISO standards

ISO 9141: Road vehicles — Diagnostic systems. International Organization for Standardization, 1989.

Part 1: Requirements for interchange of digital information
Part 2: CARB requirements for interchange of digital information
Part 3: Verification of the communication between vehicle and OBD II scan tool

ISO 11898: Road vehicles — Controller area network (CAN). International Organization for Standardization, 2003.

Part 1: Data link layer and physical signalling
Part 2: High-speed medium access unit
Part 3: Low-speed, fault-tolerant, medium-dependent interface
Part 4: Time-triggered communication

ISO 14230: Road vehicles — Diagnostic systems — Keyword Protocol 2000, International Organization for Standardization, 1999.

Part 1: Physical layer
Part 2: Data link layer
Part 3: Application layer
Part 4: Requirements for emission-related systems

ISO 15765: Road vehicles — Diagnostics on Controller Area Networks (CAN). International Organization for Standardization, 2004.

Part 1: General information
Part 2: Network layer services
Part 3: Implementation of unified diagnostic services (UDS on CAN)
Part 4: Requirements for emissions-related systems

Future developments An OBD-III specification is in the regulatory development phase. Information on the content of this specification is limited.

References

Birnbaum, Ralph and Truglia, Jerry. Getting to Know OBD II. New York, 2000. ISBN 0-9706711-0-5.

Society of Automotive Engineers International. On-Board Diagnostics for Light and Medium Duty Vehicles Standards Manual. Pennsylvania, 2003. ISBN 0-7680-1145-0.

External links

Society of Automotive Engineers International
Stern Tech's Open source OBDII scanner with schematics.
OBD2 plugs Information about OBD2 plugs.
MUTII/OBDII Diagnostics Open Mitsubishi scanner with source and schematics.
OBD-II code readers: Listen when your car speaks Introduction to OBD-II with software and hardware recommendations
Scan tool basics explained
OBD-II Trouble Codes Detailed information on OBD-II trouble codes and diagnosis information and OBD codes (ISO 9141) explained.
ScanTool.net Knowledgebase Useful information for ElmScan-compatible interfaces, as well as general OBDII information.
OBD-II Glossary Terms that you are likely to encounter in using OBD II devices.
Extensive information for OBD-II Mode 1 and 2 PIDs
Introduction to OBD protocols from KBM SystemsIncludes a list of which vehicles use which OBD protocol
National OBD Clearinghouse's Vehicle OEM DatabaseConnector locator tool

1970: The United States Congress passes the Clean Air Act and establishes the United States Environmental Protection Agency.
~1980: On-board computers begin appearing on consumer vehicles, largely motivated by their need for real-time tuning of fuel injection systems. Simple OBD implementations appear, though there is no standardization in what is monitored or how it is reported.
1982: General Motors implements an internal standard for its OBD called the Assembly Line Communications Link (ALCL), later renamed the Assembly Line Diagnostics Link (ALDL). The initial ALCL protocol communicates at 160 baud with Pulse-width modulation (PWM) signaling and monitors very few vehicle systems.
1986: An upgraded version of the ALDL protocol appears which communicates at 8192 baud with half-duplex UART signaling. This protocol is defined in GM XDE-5024B.
~1987: The California Air Resources Board (CARB) requires that all new vehicles sold in California starting in manufacturer's year 1988 (MY1988) have some basic OBD capability. The requirements they specify are generally referred to as the "OBD-I" standard, though this name is not applied until the introduction of OBD-II. The data link connector and its position are not standardized, nor is the data protocol.
1988: The Society of Automotive Engineers (Society of Automotive Engineers) recommends a standardized diagnostic connector and set of diagnostic test signals.
~1994: Motivated by a desire for a state-wide Automobile emissions control 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 connector suggested by the Society of Automotive Engineers are incorporated into this specification.
1996: The OBD-II specification is made mandatory for all cars sold in the United States.
2001: The European Union makes EOBD , a variant of OBD-II, mandatory for all petrol vehicles sold in the European Union, starting in MY2001 (see European emission standards Directive 98/69/EC ).
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 (computing)).