{{:obd-symbol.jpg?nolink | OBD symbol}} On-Board-Diagnosis (**OBD**), is used to monitor and [[diagnose]] control units in vehicles.

The functions are monitored for errors, and in the event of an error, these are stored in an [[fault log]], [[measurement values]] and setting options are made available.
This enables the workshop to read out any errors that have occurred ([[fault codes]]) from the respective error memories and to check the correct function via [[measurement values]].

An OBD system consists of at least one control unit that uses measured values from various sensors (e.g. oxygen sensors) to control actuators (e.g. injection nozzles) to achieve the desired behaviour. A modern vehicle can contain countless ECUs, each supporting hundreds of parameters that can be accessed via the DLC (Diagnostic Link Connector, [[obd-2 plug]]) using an OBD diagnostic device ([[OBD-Scanner]]).

Apps are available for mobile devices that display real-time data from the respective ECUs (virtual instrument panel). One example is the OBD app Drivedeck. For Microsoft Windows, for example, there is the OBD-2 software KOBD2Check and [[VCDS]] for manufacturer-specific OBD on Audi, Seat, Skoda and VW. An [[OBD-Scanner]] with its own display, which is independent of basic devices such as mobile phones, PCs or laptops, is practical for [[diagnosis]] in the workshop.

The readout of errors or measured values in the respective [[control unit]] via OBD is usually done by [[CAN]] bus or [[K-Line]] , in older vehicles more rarely also by PWM (pulse width modulation) or VPW (variable pulse width). Next-generation vehicles use Ethernet ([[DoIP]] ).
From 2021, models from Audi, Seat, Skoda and Volkswagen will use the mechanism [[SFD]] to regulate access to control units.

[[OBD-2]] stands for On Board Diagnostics of the second development stage. OBD-2 is available as [[EOBD]] in the EU for the following vehicles:
  * from 1.1.2000 for petrol cars
  * from 1.1.2003 for [[https://community.dieselschrauber.org/en/|Diesel]] and gas passenger cars
  * from 1.1.2005 for trucks
  * from 1.1.2017 for motorbikes

Corresponding vehicles have a [[obd-2 plug]] (OBD-2 socket) for connecting a diagnostic system. However, the socket itself is not an indication of the OBD-2 capability of the vehicle, because it was often also used on vehicles that only support manufacturer-specific diagnostics.

Efforts are being made to harmonise emission-related diagnostics worldwide; the corresponding standard [[WWH-OBD]] has been in development since 2014.

Various providers offer [[obd-help]] , which goes beyond the content of this wiki. [[https://shop.dieselschrauber.org/en/|OBD-Scanner and VCDS]]