On-Board Diagnostics, or
OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic capability. If the vehicle's onboard diagnostic system detects a malfunction, a
DTC (diagnostic trouble code) corresponding to the malfunction is stored in the vehicle's computer, and in certain cases will illuminate the
MIL (malfunction indicator light, or check engine light). A service technician can retrieve the DTC, using a "scan tool", and take appropriate action to resolve the malfunction.
Prior to the advent of digital powertrain control modules which enabled the OBD feature, repairing a vehicle relied solely upon the technician's skill and service literature from the auto manufacturer.
Overview
A modern vehicle has several hundred DTC's, each of which address a particular malfunction. A simple malfunction may be that an electrical connector is disconnected, or the fuel cap is not properly installed. A DTC may indicate a component has worn out or failed. Some DTC's indicate an entire sub-system, such as the catalyst system, is not functioning properly.
- Prior to OBD, auto manufacturers did not standardize DTC's.
- OBD-I begins standardized DTC's c. 1989
- OBD-II adds specific tests to determine the vehicle's emission performance, c. 1996
- OBD-III adds more features, and is in the regulatory development phase.
The
CARB (California Air Resources Board) is the leading activity for OBD regulations. The US EPA, the European Union, Japan, and other governments sometimes have unique OBD requirments as well. The
SAE (Society of Automotive Engineers) standardizes the DTC's among the world's auto manufacturers. Additionally, each manufacturer can further enhance the OBD capability by adding "manufacturer specific" DTC's.
Goal
The regulatory intent of OBD is to force the auto manufacturers to design reliable emission control systems that remain effective for the vehicle's "useful life". Each vehicle's emission performance deteriorates, and the rate of deterioration varies widely depending on a number of factors such as, age, mileage, usage, geography and design. The most sophisticated features of an OBD-II system determine if the vehicle's emission performance has deteriorated below a regulated level of performance; if so, the MIL will illuminate. In California, USA for instance, renewal of a vehicle's registration will be denied if the MIL is on. In the USA (beginning in 1996), component failures that illuminate the MIL are the manufacturer's responsibility to repair, provided the vehicle is within the emission warranty period. Each manufacturer is free to employ its own engineering skill to best satisfy the regulations.
It is cost prohibitive for auto manufacturers to design an emission control system that is "deterioration proof".
OBD-II Technical Specification & Interface
OBD-II is a standardized interface to the on-board computer of a vehicle. An OBD-II interface allows for the readout of DTCs (Diagnostic Trouble Codes) that have been generated by the on-board computer, as well as realtime data from the sensors connected to the on-board computer. In addition, the OBD-II interface provides a means to clear the DTC's once maintenance has been completed.
For a list of generic OBD-II DTCs, see Table of OBD-II Codes. Individual manufactures often enhance the OBD-II code set with additional proprietary DTCs.
OBD-II Physical Layer
The OBD-II specification provides for a standarized hardware interface—the female 16-pin (2x8) J1962 connector. Unlike the OBD-I connector, which was found under the hood of the vehicle, the OBD-II connector is located on the driver's side of the passenger compartment near the center console. SAE J1962 defines the pinout of the connector as:-
- —
- Bus positive Line of SAE-J1850
- —
- Chassis ground
- Signal ground
- CAN_H line of ISO 15765-4
- K line of ISO 9141-2 and ISO 14230-4
- —
- —
- Bus negative Line of SAE-J1850
- —
- —
- —
- CAN_L of ISO 15765-4
- L line of ISO 9141-2 and ISO 14230-4
- Permanent positive voltage
The assignment of unspecified pins is left to the vehicle manufacturer's discretion.
OBD-II Signal Protocols
There are five protocols in use with the OBD-II interface, and often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:
- SAE J1850 PWM (41.6 kbaud, standard of the Ford Motor Company)
- pin 2: Bus-
- pin 10: Bus+
- High voltage is +5V
- 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 kbaud, standard of General Motors)
- pin 2: Bus+
- Bus idles low
- High voltage is +7V
- Decision point is +3.5V
- Message length is restricted to 12 bytes, including CRC
- Employs CSMA/NDA
- ISO 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 12 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 CAN (250kbit/sec or 500kbit/sec)
- 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 you cannot distinguish between the two simply by examining the connector.
Diagnostic data available via OBD-II
OBD-II provides access to numerous data from the 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 formulae to convert raw OBD-II output to meaningful diagnostic units, see OBD-II PIDs.
EOBD and JOBD
In Europe the EOBD (European On-Board Diagnostics) system was mandated by European Directive 98/69/EC for all petrol vehicles made from 1 January 2001. It is similar to the American OBD-II standard. In Japan, the JOBD system is used.
ISO15765-4 (CAN)
Some newer cars (usually post-2003) also support the CAN bus. By 2008, all vehicles sold in the US must use ISO15765-4 (a variant of CAN), which will replace all other protocols for legislated diagnostics. Vehicles sold in the United States were not allowed to use CAN for diagnostics prior to model year 2004.
OBDII Scan Tools
OBDII scan tools can be categorized in two ways, based on whether they require a computer to operate (stand-alone vs PC-based), and the intended market (professional or hobby/consumer use). Thus, all scan tools fall into one of the following four categories:
- Stand-alone
- Professional
- Hobby/Consumer
- PC-based
- Professional
- Hobby/Consumer
PC-Based Scan Tools
The advantages of PC-based scan tools are:
- Low cost (compared to stand-alone scan tools with similar functionality)
- Virtually unlimited storage capacity for data logging and other functions
- Higher resolution screen
- Availability of multiple software programs
- Some are capable of reprogramming
