Technical Topics No. 19

OBD-II (On Board Diagnostics - 2nd Generation)

WHAT IS OBD-II?
OBD-II stands for On Board Diagnostics, 2nd generation. It is a set of documents issued by SAE and ISO, which describe the interchange of digital information between the on-board emission-related Electronic Control Units (ECUs) of road vehicles and an OBD-II scan tool. OBD-II also commonly refers to the physical on-board diagnostic system of a vehicle, which consists of an ECU, Malfunction Indicator Light (MIL) Diagnostic Link Connector (DLC), and the wiring that connects the different elements.

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 fast digital communications port to provide realtime 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.

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 2 models OBD-IIA and OBD-IIB.

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.
2.Bus positive Line of SAE-J1850 PWM and SAE-1850 VPW
3.Ford DCL(+) Argentina, Brazil (pre OBD-II) 1997-2000, Usa, Europe, etc. Chrysler CCD Bus(+)
4.Chassis ground
5.Signal ground
6.CAN high (ISO 15765-4 and SAE-J2284)
7.K line of ISO 9141-2 and ISO 14230-4
8.-
9.-
10.Bus negative Line of SAE-J1850 PWM only (not SAE-1850 VPW)
11.Ford DCL(-) Argentina, Brazil (pre OBD-II) 1997-2000, Usa, Europe, etc. Chrysler CCD Bus(-)
12.-
13.-
14.CAN low (ISO 15765-4 and SAE-J2284)
15.L line of ISO 9141-2 and ISO 14230-4
16.Battery voltage
The assignment of unspecified pins is left to the vehicle manufacturer's discretion.

OBD-II provides access to numerous 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 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 below, or CLICK HERE to download the complete list in .pdf format.

Individual manufacturers often enhance the OBD-II code set with additional proprietary DTCs.

GENERIC OBD-II CODES.