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Wheel alignment, which is sometimes referred to as breaking or tracking, is part of standard automobile maintenance that consists of adjusting the angles of wheels to the car manufacturer specifications.^[1] The purpose of these adjustments is to reduce tire wear and to ensure that vehicle travel is straight and true (without "pulling" to one side). Alignment angles can also be altered beyond the maker's specifications to obtain a specific handling characteristic. Motorsport and off-road applications may call for angles to be adjusted well beyond normal, for a variety of reasons.

The primary angles are the basic angle alignment of the wheels relative to each other and to the car body. These adjustments are the camber, caster and toe. On some cars, not all of these can be adjusted on every wheel.

These three parameters can be further categorized into front and rear (with no caster on the rear, typically not being steered wheels). In summary, the parameters are:

• Front: Caster (left & right)
• Front: Camber (left & right)
• Front: Toe (left, right & total)
• Rear: Camber (left & right)
• Rear: Toe (left, right & total)

The secondary angles include numerous other adjustments, such as:

• SAI (Steering Axis Inclination)
• Included angle
• Toe out on turns
• Maximum Turns
• Toe curve change
• Track width difference
• Wheelbase difference
• Front ride height
• Rear ride height
• Frame angle
• Setback

Setback is the difference between right side and left side wheelbase length. It can also be measured as an angle. Setback less than the manufacturer specified tolerance (for example, about 6mm) does not affect car handling. This is because when the vehicle is turning, one wheel is ahead of the other by several centimetres and therefore the setback is negligible. There are some car models with different factory setting for right and left side wheelbase length, for various design reasons. An off-spec setback may occur because of a collision or a difference between right and left caster.

Rake is the difference between the front and rear ride heights, a positive number when the rear ride height is larger.

A camera unit (sometimes called a "head") is attached to a specially designed clamp which holds on to a wheel. There are usually four camera units in a wheel alignment system (a camera unit for each wheel). The camera units communicate their physical positioning with respect to other camera units to a central computer, which calculates and displays. ^[2]

Often with alignment equipment, these "heads" can be a large precision reflector. In this case, the alignment "tower" contains the cameras as well as arrays of LEDs. This system flashes one array of LEDs for each reflector, whilst a camera centrally located in the LED array "looks for" an image of the reflectors patterned face. These cameras perform the same function as the other style of alignment equipment, yet alleviate numerous issues prone to relocating a heavy precision camera assembly on each vehicle serviced. ^[2]

Camber is the angle which the vertical axis of the wheel makes with the vertical axis of the vehicle. This angle is very important for the cornering performance of the vehicles. Generally, a Camber around 0.5-2 degrees is given on the vehicles. Depending upon wheel orientation, Camber can be of three types.

1. Positive Camber

The Camber would be called positive when the top of the wheels lean outwards. Positive Camber is generally used in off-road vehicles as it improves steering response and decreases steering effort. Positive Camber is also used in load-carrying vehicles. This is because the heavy load on these vehicles causes outward-leaning wheels to straighten up, improving the vehicle stability.

2. Zero Camber

The vehicle is said to have zero Camber when the wheels stand perfectly straight on the ground.

3. Negative Camber

Negative Camber is encountered when the top of the wheels lean inwards. Providing Negative Camber improves the cornering performance. When the vehicle turns on a corner, it performs a circular motion. Hence, it experiences equal and opposite centripetal & centrifugal forces. The centripetal force is experienced in the form of friction on tyres. The centrifugal force experienced by the car tries to throw it away from the turning center. This increases the normal reaction on the outer wheels. Due to increase in normal reaction, the frictional force on the outer tyres also increase. This friction acts as centripetal force and tries to bend the outer tires inwards. The tires get deformed due to bending and the contact area between the wheels and the ground decreases. This in turns decreases the frictional force between the outer tires and the ground, causing the vehicle to drift during cornering. Hence a negative Camber is given to the vehicles. The negatively cambered wheels lean inwards. So during cornering when the frictional forces try to deform the outer wheels, they just simply get flat on ground, increasing the friction with the road surface.

There are several types of wheel alignment equipment systems, and each operate in different ways.^[3]

Laser-Based Systems: The Traditional Approach

Laser alignment systems represent a more traditional approach. These systems utilise fixed heads attached simply to, or hung from each wheel, projecting laser beams to measure angles and positions. While generally considered less precise than newer technologies, laser systems are often praised for their robustness and reliability, making them a suitable choice for certain applications. However, their accuracy can be more susceptible to environmental factors and not suitable where precise accuracy is needed.

CCD (Charge-Coupled Device) Systems: Computerised Wheel Alignment

CCD alignment systems introduce computer control into the wheel alignment process. These systems employ cameras, either integrated into the main unit or attached to the wheel-mounted heads, to capture measurements. The captured data is then processed by an onboard computer, comparing it against a database of manufacturer specifications. This computerised approach allows for the detection of even minor deviations from recommended settings, enhancing accuracy compared to purely laser-based systems.

3D Imaging: Giving More Accurate 4-Wheel Alignment

3D wheel alignment systems represent a step forwards in vehicle wheel alignment. Employing advanced three-dimensional imaging technology, these systems rapidly capture a comprehensive picture of wheel angles and positions. Like CCD systems, 3D aligners rely on a vehicle database for comparison and offer exceptional speed and accuracy, capable of identifying the smallest misalignments. The detailed data provided by 3D systems allows for more intricate adjustments and a more thorough understanding of the vehicle's alignment geometry, particularly suited to 4 wheel drive cars or those needed 4 wheel alignment.

Drive-over, Drive-on or Drive Through Wheel Alignment Systems: Next Generation Alignment

The most recent, and certainly most expensive wheel alignment systems employ touchless systems to analyse the wheel geometry. These aligners no longer require heads to be hung on or clamped to the car wheels, instead the car either drives over a speed-bump style device that records the wheel and tyre measurements, or other aligners all the car to be driven between pillars containing fixed cameras that take the measurements from the vehicle wheels as they drive into, or through the view of the cameras. These represent the next generation of wheel alignment systems, and whilst they may be cost prohibitive for some workshops, the throughput of vehicles may help justify the costs where the volume of vehicles being aligned may justify this.

Portable Alignment Machines: Mobility and Convenience

For mobile mechanics or workshops with limited space, portable wheel alignment machines offer a practical solution. Designed for easy transport and setup, these systems are lightweight and compact. While convenient, portable systems may not achieve the same level of accuracy as their larger, stationary counterparts. They are often chosen for their flexibility rather than ultimate precision.

• The vehicle pulls to one side
• The steering wheel does not return to center
• The steering wheel is off-center
• Excessive tire wear in certain spots
• Loose steering

 

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.^[1][2]

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.

This section is in list format but may read better as prose. You can help by converting this section, if appropriate. Editing help is available. (September 2021)

• 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.^[3]
• 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.^[4][5]
• 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.^[6]
• 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.)^[7][8] 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 8,500 lb (3,900 kg) 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 14,000 lb (6,400 kg).^[8]
• 1997: The OBD-II specification is made mandatory for California emissions diesel-engined vehicles with a gross vehicle weight rating up to 14,000 lb (6,400 kg).^[8]
• 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^[9]).
• 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 14,000 lb (6,400 kg) are required to have OBD-II.^[8]
• 2006: All vehicles manufactured in Australia and New Zealand are required to be OBD-II compliant after January 1, 2006.^[10] All vehicles in the United States of 14,000 lb (6,400 kg) gross vehicle weight rating and under are required to have OBD-II.^[8]
• 2007: All California emissions vehicles over 14,000 lb (6,400 kg) 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^[11] signaling standard (a variant of the Controller Area Network (CAN) bus).^[12]
• 2008: Certain light vehicles in China are required by the Environmental Protection Administration Office to implement OBD (standard GB18352^[13]) by July 1, 2008.^[14] Some regional exemptions may apply.
• 2010: Start of required phase-in of the OBD-II specification to all vehicles with a gross vehicle weight rating of 14,000 lb (6,400 kg) and above, 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+.^[8]

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.^[15][16]

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".^[17] The hope was that by forcing annual emissions testing for California starting in 1988, ^[18] 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.^[19]

The Diagnostic Trouble Codes (DTC's) 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 DTC's from the vehicle. DTC's 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 DTC's 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 DTC's available than OBD-II equipped vehicles.

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.^[20]

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.^[21]

An OBD 1.5 has also been used in the Ford Scorpio since 95.^[22]

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.

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 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. Audi: Switched +12 to tell a scan tool whether the ignition is on. VW: Switched +12 to tell a scan tool whether the ignition is on. Mercedes[23] (K-Line): Ignition control (EZS), air-conditioner (KLA), PTS, safety systems (Airbag, SRS, AB) and some other.	9	Manufacturer discretion GM: 8192 baud ALDL where fitted. BMW: RPM signal. Toyota: RPM signal. Mercedes (K-Line): ABS, ASR, ESP, ETS, BAS diagnostic.
2	Bus positive Line SAE J1850 PWM and VPW	10	Bus negative Line SAE J1850 PWM only (not SAE 1850 VPW)
3	Manufacturer discretion Ethernet TX+ (Diagnostics over IP) Ford DCL(+) Argentina, Brazil (pre OBD-II) 1997–2000, USA, Europe, etc. Chrysler CCD Bus(+) Mercedes (TNA): TD engine rotation speed.	11	Manufacturer discretion Ethernet TX- (Diagnostics over IP) Ford DCL(-) Argentina, Brazil (pre OBD-II) 1997–2000, USA, Europe, etc. Chrysler CCD Bus(-) Mercedes (K-Line): Gearbox and other transmission components (EGS, ETC, FTC).
4	Chassis ground	12	Manufacturer discretion Ethernet RX+ (Diagnostics over IP) Mercedes (K-Line): All activity module (AAM), Radio (RD), ICS (and more)
5	Signal ground	13	Manufacturer discretion Ethernet RX- (Diagnostics over IP) Ford: FEPS – Programming PCM voltage Mercedes (K-Line): AB diagnostic – safety systems.
6	CAN high (ISO 15765-4 and SAE J2284)	14	CAN low (ISO 15765-4 and SAE J2284)
7	K-line (ISO 9141-2 and ISO 14230-4)	15	L-line (ISO 9141-2 and ISO 14230-4)
8	Manufacturer discretion Activate Ethernet (Diagnostics over IP) Many BMWs: A second K-line for non OBD-II (Body/Chassis/Infotainment) systems. Mercedes: Ignition	16	Battery voltage (+12 Volt for type A connector) (+24 Volt for type B connector)

The assignment of unspecified pins is left to the vehicle manufacturer's discretion.^[24]

The European on-board diagnostics (EOBD) 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 2,500 kg, 5,500 lb or less) first registered within EU member states since January 1, 2001 for petrol-engined cars and since January 1, 2004 for diesel engined cars.^[25]

For newly introduced models, the regulation dates applied a year earlier – January 1, 2000 for petrol and January 1, 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 January 1, 2002, for petrol models, and January 1, 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 are lower than previous Euro III and IV.

Each of the EOBD fault codes consists of five characters: a letter, followed by four numbers.^[26] 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 next character would refer to the sub system.

• P00xx – Fuel and Air Metering and Auxiliary Emission Controls.
• P01xx – Fuel and Air Metering.
• P02xx – Fuel and Air Metering (Injector Circuit).
• P03xx – Ignition System or Misfire.
• P04xx – Auxiliary Emissions Controls.
• P05xx – Vehicle Speed Controls and Idle Control System.
• P06xx – Computer Output Circuit.
• P07xx – Transmission.
• P08xx – Transmission.

The following two characters would refer to the individual fault within each subsystem.^[27]

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) is the Australian equivalent of OBD-II. It applies to all vehicles of category M1 and N1 with a Gross Vehicle Weight rating of 3,500 kg (7,700 lb) or less, registered from new within Australia and produced since January 1, 2006 for petrol-engined cars and since January 1, 2007 for diesel-engined cars.^[28]

For newly introduced models, the regulation dates applied a year earlier – January 1, 2005 for petrol and January 1, 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 July 1, 2008, for new models, July 1, 2010, for all models.^[29]

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.

In North America, EMD and EMD+ are on-board diagnostic systems that were used on vehicles with a gross vehicle weight rating of 14,000 lb (6,400 kg) or more between the 2007 and 2012 model years if those vehicles did not already implement OBD-II. EMD was used on California emissions vehicles between model years 2007 and 2009 that did not already have OBD-II. EMD was required to monitor fuel delivery, exhaust gas recirculation, the diesel particulate filter (on diesel engines), and emissions-related powertrain control module inputs and outputs for circuit continuity, data rationality, and output functionality. EMD+ was used on model year 2010-2012 California and Federal petrol-engined vehicles with a gross vehicle weight rating of over 14,000 lb (6,400 kg), it added the ability to monitor nitrogen oxide catalyst performance. EMD and EMD+ are similar to OBD-I in logic but use the same SAE J1962 data connector and CAN bus as OBD-II systems.^[8]

Five signaling protocols 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:^[30]

• SAE J1850 PWM (pulse-width modulation — 41.6 kB/sec, standard of the Ford Motor Company)
  • pin 2: Bus+
  • pin 10: Bus–
  • High voltage is +5 V
  • Message length is restricted to 12 bytes, including CRC
  • 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 kB/sec, standard of General Motors)
  • pin 2: Bus+
  • Bus idles low
  • High voltage is +7 V
  • Decision point is +3.5 V
  • Message length is restricted to 12 bytes, including CRC
  • Employs CSMA/NDA
• ISO 9141-2^[31] This protocol has an asynchronous serial data rate of 10.4 kbit/s.^[32] It is somewhat similar to RS-232; however, the signal levels are different, and communications happen on a single, bidirectional line without additional handshake signals. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
  • pin 7: K-line
  • pin 15: L-line (optional)
  • UART signaling
  • K-line idles high, with a 510 ohm resistor to V_batt
  • The active/dominant state is driven low with an open-collector driver
  • Message length is max 260 Bytes (payload field max is 255 Bytes)
• 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 CAN (250 kbit/s or 500 kbit/s). The CAN protocol was developed by Bosch for automotive and industrial control. Unlike other OBD protocols, variants are widely used outside of the automotive industry. While it did not meet the OBD-II requirements for U.S. vehicles prior to 2003, as of 2008 all vehicles sold in the US are required to implement CAN as one of their signaling protocols.
  • pin 6: CAN High
  • pin 14: CAN Low

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 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 available parameters 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. In SAE J1979 these "modes" were renamed to "services", starting in 2003.

• Service / Mode $01 shows current sensor live data from PIDs ("Parameter IDs"). See OBD-II PIDs#Service_01 for an extensive list.
• Service / Mode $02 makes Freeze Frame data accessible via the same PIDs.^[33] See OBD-II PIDs#Service_02 for a list.
• Service / Mode $03 lists the emission-related "confirmed" diagnostic trouble codes stored. It either displays numeric, 4 digit codes identifying the faults or maps them to a letter (P, B, U, C) plus 4 digits. See #OBD-II_diagnostic_trouble_codes.
• Service / Mode $04 is used to clear emission-related diagnostic information. This includes clearing the stored pending/confirmed DTCs and Freeze Frame data.^[34]
• Service / 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
  • See OBD-II PIDs#Service_05 for a list.
• Service / 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.
• Service / 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. See #OBD-II_diagnostic_trouble_codes.
• Service / Mode $08 could enable the off-board test device to control the operation of an on-board system, test, or component.
• Service / Mode $09 is used to retrieve vehicle information. Among others, the following information is available:
  • VIN (Vehicle Identification Number): Vehicle ID
  • CALID (Calibration Identification): ID for the software installed on the ECU
  • CVN (Calibration Verification Number): Number used to verify the integrity of the vehicle software. The manufacturer is responsible for determining the method of calculating CVN(s), e.g. using checksum.
  • In-use performance counters
    • Petrol engine : Catalyst, Primary oxygen sensor, Evaporating system, EGR system, VVT system, Secondary air system, and Secondary oxygen sensor
    • Diesel engine : NMHC catalyst, NOx reduction catalyst, NOx absorber Particulate matter filter, Exhaust gas sensor, EGR system, VVT system, Boost pressure control, Fuel system.
  • See OBD-II PIDs#Service_09 for an extensive list.
• Service / 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. See #OBD-II_diagnostic_trouble_codes.

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.

• Simple fault code readers/reset tools are mostly aimed at the consumer level.
• Professional hand-held scan tools may possess more advanced functions
  • Access more advanced diagnostics
  • Set manufacturer- or vehicle-specific ECU parameters
  • Access and control other control units, such as air bag or ABS
  • Real-time monitoring or graphing of engine parameters to facilitate diagnosis or tuning

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 or Bluetooth adapters plugged into the car's OBD II connector. Newer devices on the market are equipped with GPS sensors and the ability to transmit vehicle location and diagnostics data over a cellular network. Modern OBD-II devices can therefore nowadays be used to for example locate vehicles, monitor driving behavior in addition to reading Diagnostics Trouble Codes (DTC). Even more advanced devices allow users to reset engine DTC codes, effectively turning off engine lights in the dashboard; however, resetting the codes does not address the underlying issues and can in worst-case scenarios even lead to engine breakage where the source issue is serious and left unattended for long periods.^[36][37]

An OBD-II software package when installed in a computer (Windows, Mac, or Linux) can help diagnose the onboard system, read and erase DTCs, turn off MIL, show real-time data, and measure vehicle fuel economy.^[38]

To use OBD-II software, one needs to have an OBD-II adapter (commonly using Bluetooth, Wi-Fi or USB)^[39] plugged in the OBD-II port to enable the vehicle to connect with the computer where the software is installed.^[40]

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 ELM327 or STN^[41] 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:

• Large storage capacity for data logging and other functions
• Higher resolution screen than handheld tools
• The ability to use multiple software programs adding flexibility
• The identification and clearance of fault code
• Data shown by intuitive graphs and charts

The extent that a PC tool may access manufacturer or vehicle-specific ECU diagnostics varies between software products^[42] 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:

• Engine and vehicle monitoring under normal operation, for diagnosis or tuning.
• Some US auto insurance companies offer reduced premiums if OBD-II vehicle data loggers^[43][44] or cameras are installed – and if the driver's behaviour meets requirements. This is a form of auto insurance risk selection
• Monitoring of driver behaviour by fleet vehicle operators.

Analysis of vehicle black box data may be performed periodically, 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.^[45]

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,^[46] carputer or interfaces to PDAs,^[47] smartphones, or a Garmin navigation unit.

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 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. It can be used to detect reckless driving in real time based on the sensor data provided through the OBD port.^[48] This detection is done by adding a complex events processor (CEP) to the backend and on the client's interface. OBD II is also monitored to block mobile phones when driving and to record trip data for insurance purposes.^[49]

OBD-II diagnostic trouble codes (DTCs)^[50][51] are five characters long, with the first letter indicating a category, and the remaining four being a hexadecimal number.^[52]

The first character, representing category can only be one of the following four letters, given here with their associated meanings. (This restriction in number is due to how only two bits of memory are used to indicate the category when DTCs are stored and transmitted).^[52]

• P – Powertrain (engine, transmission and ignition)
• C – Chassis (includes ABS and brake fluid)
• B – Body (includes air conditioning and airbag)
• U – Network^[a] (wiring bus)

The second character is a number in the range of 0–3. (This restriction is again due to memory storage limitations).^[52]

• 0 – Indicates a generic (SAE defined) code
• 1 – Indicates a manufacturer-specific (OEM) code
• 2 – Category dependent:
  • For the 'P' category this indicates a generic (SAE defined) code
  • For other categories indicates a manufacturer-specific (OEM) code
• 3 – Category dependent:
  • For the 'P' category this is indicates a code that has been 'jointly' defined
  • For other categories this has been reserved for future use

The third character may denote a particular vehicle system that the fault relates to.^[50]

• 0 – Fuel and air metering and auxiliary emission controls
• 1 – Fuel and air metering
• 2 – Fuel and air metering (injector circuit)
• 3 – Ignition systems or misfires
• 4 – Auxiliary emission controls
• 5 – Vehicle speed control and idle control systems
• 6 – Computer and output circuit
• 7 – Transmission
• 8 – Transmission
• A-F – Hybrid Trouble Codes

Finally the fourth and fifth characters define the exact problem detected.

• 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 CAN physical and data link layer
• J2411 – Describes the GMLAN (Single-Wire CAN) protocol, used in newer GM vehicles. Often accessible on the OBD connector as PIN 1 on newer GM vehicles

• J1939 – Defines a data protocol for heavy duty commercial vehicles

• 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 15031: Communication between vehicle and external equipment for emissions-related diagnostics, International Organization for Standardization, 2010.
  • Part 1: General information and use case definition
  • Part 2: Guidance on terms, definitions, abbreviations and acronyms
  • Part 3: Diagnostic connector and related electrical circuits, specification and use
  • Part 4: External test equipment
  • Part 5: Emissions-related diagnostic services
  • Part 6: Diagnostic trouble code definitions
  • Part 7: Data link security
• ISO 15765: Road vehicles – Diagnostics on Controller Area Networks (CAN). International Organization for Standardization, 2004.
  • Part 1: General information
  • Part 2: Network layer services ISO 15765-2
  • Part 3: Implementation of unified diagnostic services (UDS on CAN)
  • Part 4: Requirements for emissions-related systems

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.^[53][54][55]

There have been reports of thieves using specialist OBD reprogramming devices to enable them to steal cars without the use of a key.^[56] 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.^[57]

• OBD-II PIDs ("Parameter IDs")
• Unified Diagnostic Services
• Engine control unit
• Immobiliser

A motor vehicle service or tune-up is a series of maintenance procedures carried out at a set time interval or after the vehicle has traveled a certain distance. The service intervals are specified by the vehicle manufacturer in a service schedule and some modern cars display the due date for the next service electronically on the instrument panel. A tune-up should not be confused with engine tuning, which is the modifying of an engine to perform better than the original specification, rather than using maintenance to keep the engine running as it should.

• Inspection - vehicle components are visually inspected for wear or any leaks. A diagnostic is performed to identify any electrical components reporting a failure or a part operating outside of normal conditions.
• Replacement - Given certain lubricants break down over time due to heat and wear, manufacturers recommend replacement. Any parts that are close to their expected failure are replaced too to avoid a failure while operating the vehicle.
• Adjustments - as vehicle components wear, they may need adjustment over time. Example: parking brake cable.

The completed services are usually recorded in a service book or digital service record upon completion of each service. A digital service record is an online record of a vehicle's maintenance history.^[1] A complete service history usually adds to the resale value of a vehicle.

Difference between major and full service: a major service is more comprehensive than a full service; although it covers all the same checks that a full service does, a major service will be more detailed and will include more replacements of wearable parts, such as pollen filters, and changing brake fluid if required.

As a guideline, minor car services are carried out every 10,000 to 15,000 kilometres (6,200 to 9,300 miles), and major car services every 30,000 to 45,000 kilometres (19,000 to 28,000 miles) – or every twelve months, whichever comes first.

The actual schedule of car maintenance varies depending on the year, make, and model of a car, its driving conditions, and driver behavior. Carmakers recommend the so-called extreme or the ideal service schedule based on impact parameters such as

• the number of trips and distance traveled per trip per day
• extreme hot or cold climate conditions
• mountainous, dusty, or DE-iced roads
• heavy stop-and-go vs. long-distance cruising
• towing a trailer or other heavy load

Service advisers in dealerships and independent shops recommend schedule intervals, which are often in between the ideal or extreme service schedule.

In addition, drivers may be penalized for not regularly servicing their cars. For example, in many states in the U.S., a car has to pass a safety inspection test every year or two years to remain legal, and can incur fines for continuing to drive cars that have failed.^[2]

Maintenance tasks commonly carried out during a motor vehicle service include:

Mechanical parts that may cause the car to cease transmission or prove unsafe for the road are also noted and advised upon.

In the United Kingdom, few parts that are not inspected on the MOT test are inspected and advised upon a Service Inspection, including clutch, gearbox, car battery, and engine components (further inspections than MOT).

• Auto mechanic
• Automobile repair shop
• Bus garage
• Car ramp, a means of accessing the underside of a vehicle
• Engine tuning
• Exhaust gas analyzer
• Italian tuneup
• Mechanical engineering

• Hillier, V.A.W.; Coombes, P. (2004). Hillier's Fundamentals of Motor Vehicle Technology. Nelson Thornes. ISBN 978-0-7487-8082-2. Retrieved 24 May 2021.

The examples and perspective in this article may not represent a worldwide view of the subject. You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate. (July 2020) (Learn how and when to remove this message)

Roadworthiness^[1] or streetworthiness is a property or ability of a car, bus, truck or any kind of automobile to be in a suitable operating condition or meeting acceptable standards for safe driving and transport of people, baggage or cargo in roads or streets, being therefore street-legal.

In Europe, roadworthy inspection is regulated by:

• Directive 2014/45/EU, on periodic roadworthiness tests for motor vehicles and their trailers,^[2]
• Directive 2014/46/EU, on the registration documents for vehicles,^[3]
• Directive 2014/47/EU, on the technical roadside inspection of the roadworthiness of commercial vehicles.^[4][5]

A Certificate of Roadworthiness (also known as a ‘roadworthy’ or ‘RWC’) attests that a vehicle is safe enough to be used on public roads. A roadworthy is required in the selling of a vehicle in some countries. It may also be required when the vehicle is re-registered, and to clear some problematic notices.^[6]

"roadworthiness certificate" means a road-worthiness test report issued by the competent authority or a testing centre containing the result of the road-worthiness test

— DIRECTIVE 2014/45/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 3 April 2014 on periodic roadworthiness tests for motor vehicles and their trailers and repealing Directive 2009/40/EC

Roadworthy inspection is designed to check the vehicle to make sure that its important auto parts are in a good (not top) condition that is enough for safe road use. It includes:^[6]

• mirrors
• wheels and tires
• vehicle structure
• lights and reflectors
• seats and seat belts
• steering, suspensions and braking systems
• windscreen, and windows including front wipers and washers
• other safety related items on the body, chassis or engine

Roadworthy inspection in Europe

Directive 2014/45/EU regulates the periodic testing for various kind of vehicles:

• transport of people (M1, M2, M3)
• transport of good (N1, N2, N3)
• trailers of more than 3.5 tonnes (O3, O3)
• tractors of category T5
• since January 2022, two- or three-wheel vehicles in categories L3e, L4e, L5e and L7e, with an engine displacement of more than 125 cm3.^[2]

18 of 27 EU member states have required motorcycle owners to have their vehicles checked for road-worthiness. The directive 2014/45/EU defines obligations and responsibilities, minimum requirements concerning road-worthiness tests, administrative provisions and cooperation and exchange of information.

Minimum requirements concerning road-worthiness tests encompass date and frequency of testing, contents and methods of testing, assessment of deficiencies, road-worthiness certificate, follow-up of deficiencies and proof of test.^[2]