On-board diagnostics

On- board diagnostics ( OBD) is a vehicle diagnostic system. During driving operation, all emission- affecting systems are monitored, in addition another important control devices whose data can be accessed through their software. Errors are displayed to the driver via a warning light and stored in the respective control unit permanently. Error messages can then be retrieved later by a specialist garage with standardized interfaces. The codes (called P0 codes ) are defined in the ISO standard 15031-6.

Origin and basic idea

OBD systems were introduced in 1988 by the California Air Resources Board, California ( the "Authority for the Clean Air " ) in the United States. This was based on the consideration that it is not sufficient to comply with the emission requirements in the authorization but that the observance of the life to be assured. The OBD - I standard provides that the vehicle has its own electronic systems for self-monitoring. These have emission-related fault on a built- in fittings signal lamp - the so-called engine check light (MIL = " Malfunction Indicator Light " ) - Show. In addition, errors in one with onboard tools (flash code ) readable memory must be stored.

The latest regulations shall be a monitoring of the monitoring function. It is based on the fear that the diagnoses over the lifetime are not performed regularly. Therefore, must be recorded how often the diagnoses are carried out, and be certain quotas set ( IUMPR: In use monitor performance ratio ). The results can have a standardized connector via a serial interface with standardized protocol (see K-line ) or be read out via the CAN bus.


The presence of a engine control light in view of the driver is required for approval of new vehicles in Europe ( EOBD). This applies to cars with petrol engine from model year 2001 for cars with diesel engines from model year 2004. Particular vehicle models for the U.S. export but are also much older construction OBD (-2) -capable. The lack of OBD was initially the reason why the last built new VW Beetle from Mexico ( " Ultima Edicion " ) should not receive approval for their import to Germany.

Reads out OBD information

Access for vehicle diagnostics via OBD -2 is the 16-pin OBD-2 diagnostic socket in the vehicle, which is often used not only for cross-vendor, emissions-related OBD -2 diagnostic log, but also for the specific diagnostic protocols of the manufacturers.

As a physical interface, the K-line or CAN - bus is being used. OBD monitors include the following systems and sensors:

  • Lambda probe function
  • Misfiring by a Inkrementenrads
  • Efficiency of the catalyst

And if available also:

  • Function of the exhaust gas recirculation
  • Function of the secondary air system,
  • Tank ventilation system

Each request to the control unit consists of a mode and a data set ( parameter ID, PID) of this mode. PIDs are standardized in certain areas since OBD -2 ( SAE J1979 ).


After an error occurs, a debounce counter is first started. Not the fault disappears before the end of the debounce time, the entry takes place in the fault memory and, if necessary turning on the check engine light.

Readiness Code

Not all emission-related components can be continuously monitored, because ( for example, when catalyst) first certain operating conditions must be achieved. Based on the readiness codes can be read out using a commercially available scan tool if all emission-related components or devices have been tested by the OBD. The readiness code is read at the emission inspection AU and evaluated.

Diagnostic software

The results of the on-board diagnostics can be read by special software applications on standard notebooks. About the diagnostic interface of the repair shops or notebook computer sends (via additional hardware required for protocol interpretation and signal level conversion) commands to one of the control devices, which is activated by its address, and then gets back results. Commands have to read the ID ( precise model name and version ) of the control device for reading and resetting of the above error messages, to read of so-called measuring value blocks (also standard display called ), for reading, testing and setting of various setting parameters ( so-called adaption channels ) and (especially for development) for direct reading and writing of memory cells in the control unit.

For smartphone apps are available that allow an adapter with an OBD diagnostics.


The diagnostics are divided into different groups.

  • Short circuit to ground
  • Short circuit to battery
  • Cable break
  • Implausible voltage
  • Plausibility diagnosis ( value of a sensor is located in the permitted range of the current operating status )
  • Balance diagnostics (several sensors are compared )
  • " Stuck" diagnosis ( stuck? ): Modifies the value at transient conditions?
  • Gradient monitoring (checking whether the rise of a sensor signal is real possible)

Further benefits

In addition to the security and protection of the environment, the OBD is designed to prevent engine damage in practice: the case of corresponding errors then motor saving emergency operation can be activated. For example: After detecting a loose spark plug cable ( " cable break " ) the corresponding cylinder is turned off ( no fuel injected), otherwise the unburned mixture could destroy the catalyst. The driver takes it (besides the possibly flashing MIL ) is true as performance degradation.

Furthermore, the OBD can also serve to simplify maintenance and repairs: The information from the OBD can search for the defective component easier after occurrence of a fault symptom or make it unnecessary. This requires, however, that to the respective error messages according to a detailed service documentation from the manufacturer is provided.

In addition, the diagnosis is a valuable tool during the development phase of ECUs.


About the OBD interface can not be read only data, but also general commands are sent to the vehicle. By joining, for example, with a laptop with the appropriate software may be located outside of the multi-vendor, emission-related OBD-2 diagnostic protocol by vendor-specific communication protocols such as the electronic immobilizer bypassed and the vehicle are illegal moves.