On-board diagnosis

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On-Board Diagnostics ( OBD ) is a vehicle diagnostic system . All systems influencing exhaust emissions are monitored during driving , as are other important control units , the data of which can be accessed through their software. Any errors that occur are displayed to the driver via a control lamp and permanently stored in the respective control unit. Error messages can then be queried later by a specialist workshop via standardized interfaces. Initially, the data was recorded and evaluated according to different principles of the various manufacturers, sometimes even from model to model in a different form. In retrospect, this phase is now referred to as OBD-1 (also OBD I), while after the standardization one speaks of OBD-2 (also OBD II ). Since OBD-2, the error codes (DTC - Diagnostic Trouble Code), also called P0 codes, are defined in the SAE J2012 and ISO 15031-6 standards.

Origin and basic idea

OBD systems in 1988 from California California Air Resources Board (CARB, dt. About " clean air Authoritative Body") was introduced in the United States. The basis was the consideration that it is not sufficient to adhere to the emission regulations for approval, but that compliance should be ensured over the service life. The OBD-I standard provides that the vehicle has its own electronic systems for self-monitoring. These must indicate emissions-related errors via a signal lamp integrated in the dashboard - the so-called engine control light (MIL = "Malfunction Indicator Light"). In addition, errors must be stored in a memory that can be read out using on-board tools (blink code).

The latest regulations require monitoring of the monitoring function. The basis is the fear that the diagnoses will not be carried out regularly over the lifespan. It must therefore be recorded how often the diagnoses are carried out and certain quotas are specified (IUMPR: In use monitor performance ratio). The results can be read out via a standardized connector via a serial interface with standardized protocol (see K-line ) or via the CAN bus.

In addition to the environmentally relevant task area of ​​emissions monitoring that was originally envisaged, other areas of vehicle diagnosis were gradually added. Safety-relevant areas such as belt systems and airbags, errors such as short circuits and line interruptions, problems with possible engine damage as a result, maintenance information such as the oil level, etc. are now integrated into the OBD.

General

In the European Union, Regulation (EC) No. 715/2007 stipulates that when a new vehicle is registered, an engine control lamp is part of the OBD. This applies to cars with gasoline engines from model year 2001 and for cars with diesel engines from model year 2004. In particular, vehicle models for export to the USA are OBD (-2) -compatible even in much older years of construction. The lack of an OBD was initially the reason why the last new VW Beetles built from Mexico (“Ultima Edicion”) should not be approved after they were imported into Germany.

Reading the OBD information

Typical USB - KKL diagnostic interface for signal level conversion without its own protocol logic, which is why the associated software must be able to handle the diagnostic protocol

Access for vehicle diagnosis via OBD-2 is the 16-pin OBD-2 diagnosis socket (CARB socket or Diagnostic Link Connector (DLC)) in the vehicle, which is often used not only for the manufacturer-independent, emissions-related OBD-2 diagnosis protocol , but also for the manufacturer's specific diagnostic protocols.

The K line or the CAN bus is used as the physical interface. OBD monitors the following systems and sensors , among others :

and if available also:

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

  • OBD-2 socket on a Peugeot 208

  • OBD-2 socket on a Mercedes-Benz

  • OBD-2 scanner with Bluetooth transmission

  • Handheld diagnostic device

  • Display of an OBD scanner

  • OBD-2 socket, plug assignment

Debouncing

When an error occurs, a debounce counter is started first. The error does not disappear before the expiration of the debounce time , the entry in occurs fault memory and optionally the switching on of the motor indicator light .

Readiness code

Not all components relevant to exhaust gas can be permanently monitored because (for example with the catalytic converter) certain operating states must first be achieved. Using the readiness code, a commercially available scan tool can be used to read out whether all components or equipment relevant to exhaust emissions have been checked by the OBD. The readiness code is read out and assessed during the investigation of the engine management and emission control system UMA.

Diagnostic software

The results of the on-board diagnosis can also be read out using special software applications on commercially available notebooks. The connected workshop or notebook computer (via the additional hardware required for protocol interpretation and signal level conversion) sends commands to one of the control units via the diagnostic interface, which is activated via its address and then receives the results back. There are commands for reading the ID (precise model name and version) of the control unit, for reading and resetting the above-mentioned error entries , for reading out so-called measured value blocks (also called standard display ), for reading, testing and setting various setting parameters (so-called adjustment channels ) and (especially for development) for direct reading and writing of memory cells in the control unit.

Apps are available for smartphones that enable wireless readout and evaluation of the OBD interface with a corresponding Bluetooth adapter.

division

The diagnoses are divided into different groups.

Electrical diagnostics (for a wide variety of lines)
  • Short circuit to ground
  • Short circuit to battery
  • Cable break
  • implausible tension
Sensor diagnostics
  • Plausibility diagnosis (value of a sensor is in the permitted range of the current operating status)
  • Adjustment diagnosis (several sensors are compared with one another)
  • “Stuck” diagnosis (stuck?): Does the value change under transient conditions?
  • Gradient monitoring (checking whether the increase in a sensor signal is actually possible)
Actuator diagnostics
Does the actuator react to an activation (measured by sensors)?
System diagnostics
Are the output values ​​of a system acceptable over a required period of time under changed conditions (is a misfiring cylinder recognized through uneven engine running)?
Component diagnostics
This area affects components that do not belong directly to the sensors / actuators and are monitored by their own or other existing sensors, tank leakage diagnosis , catalyst diagnosis or "hose burst detection".

Another benefit

In addition to averting danger and protecting the environment, the OBD is also intended to prevent engine damage in practice: In the event of corresponding errors, emergency running programs that are gentle on the engine are activated. For example, if a loose spark plug cable is detected (“cable break”), the corresponding cylinder is switched off (no fuel is injected), as otherwise the unburned mixture could destroy the catalytic converter. The driver perceives this (in addition to the possibly flashing MIL) as a drop in performance.

Furthermore, the OBD can also be used to simplify maintenance and repairs. Your information can make the search for the defective component easier or even superfluous after the occurrence of a fault symptom. The prerequisite for this, however, is that the manufacturer provides correspondingly detailed service documentation for the respective error messages.

Diagnosis is also a valuable aid during the development phase of control units.

criticism

Not only can data be read out via the OBD interface, but general commands can also be sent to the vehicle. By connecting, for example, to a notebook with the appropriate software, the electronic immobilizer can be circumvented and the vehicle can be unlawfully moved outside of the manufacturer-independent, emissions-related OBD-2 diagnostic protocol using manufacturer-specific communication protocols in some vehicle models .

Consumer advocates criticize the fact that the odometer reading on the odometer can be manipulated via the OBD-2 interface, especially in older used cars .

See also

literature

Web links

Individual evidence

  1. ↑ www.obd-2.de General information
  2. ↑ Smartphone as a car computer and diagnostic tool: OBD on the cell phone. In: NetMediaEurope Deutschland GmbH. Retrieved September 30, 2013 .
  3. ↑ Forensic testing laboratory GÖTH GmbH, Wayen. (PDF) Pages 21, 24
  4. ↑ Speedometer fraud: How buyers of used cars limit their risk , test.de from January 15, 2018, accessed on February 6, 2018