What data is available through OBD?

On-board diagnostics (OBD) is an automotive concept that refers to a vehicle’s ability to self-diagnose and report. OBD systems provide access to the status of different vehicle subsystems to the vehicle owner or repair technician.

Since the advent of on-board vehicle computers in the early 1980s, the amount of diagnostic information available via OBD has varied greatly. If a problem was found, early versions of OBD would simply illuminate a malfunction warning light or “idiot light,” but would not include any detail about the existence of the problem.

Modern OBD implementations use a standardized digital communications port to provide real-time data as well as a standardized set of diagnostic trouble codes, or DTCs, that enable an individual to quickly identify and correct vehicle malfunctions.

OBD-II provides a standard method for accessing a variety of data, including

RPM, rpm, pedal position, spark advance, airflow rate, coolant temperature, and other real-time parameters
What is the status of the “Check Engine” light?
Status of emission readiness
Freeze frame: a “snapshot” of parameters taken at the time of a problem.
Codes for troubleshooting (DTCs).
The outcome of an oxygen sensor test
Number assigned to a vehicle (VIN)
The total number of ignition cycles is
How many miles have you driven with MIL on?

OBD-II is an acronym for “On-Board Diagnostics
In terms of capability and standardisation, OBD-II outperforms OBD-I. The OBD-II standard defines the type of diagnostic connector and pinout, as well as the electrical signalling protocols and messaging format that are applicable. It also includes a list of potential vehicle parameters to track, as well as instructions on how to encode the data for each. A pin in the connector provides power to the scan tool from the vehicle battery, eliminating the need to attach a scan tool to a separate power source. Some technicians, however, may elect to attach the scan tool to an auxiliary power source in order to protect data in the unlikely event that a vehicle loses electrical power due to a malfunction. Finally, the OBD-II standard includes a number of DTCs that can be expanded. As a result of this standardisation, any vehicle’s on-board computer(s) can be queried by a single device. This OBD-II was available in two versions: OBD-IIA and OBD-IIB. While only emission-related codes and data are expected to be transmitted via the OBD-II Data Link Connector, most manufacturers have made it the only one in the vehicle through which all systems are diagnosed and configured. P for engine and transmission (powertrain), B for body, C for chassis, and U for network are the four-digit OBD-II Diagnostic Trouble Codes.

Diagnostic adapter OBD-II

On a vehicle, there is a female OBD-II connector.

Pinout of a female OBD-II connector seen from the front
The female 16-pin (2×8) J1962 connector is a generic hardware interface in the OBD-II specification. The OBD-II connector, unlike the OBD-I connector, which was often located under the hood of the car, must 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).