On-Board Diagnostics, or OBD, is a 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.. 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.
The OBD-II standard specifies the type of diagnostic connector and its pin-out, the electrical signaling protocols available, and the messaging format. The OBD-II standard provides an extensible list of DTC's (Diagnostic Trouble Codes). 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 (power-train), 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 pin-out 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