On Board Diagnosis (additional  
    information)

Function

Without additional measuring instruments the On board diagnostic allows an actual value comparison of what it is now and what it should be. Besides, not only the exhaust gas values are controlled permanently but also other engine functions. In case of disturbances, e.g., in the ignition system, the control device can interrupt and even switch off single cylinders. During the exhaust gas emission test the garage tester is only used if it is equipped with an additional scanner function. Only the single areas are read.

How it works

The examination of the exhaust gas values in the running mode does not take place directly by checking the four measurements, but by more exact analysis of the signals of the pre-catalyst and post-catalyst sensor, the sensor for the engine speed, the knock sensor, the acceleration transducer (ESP) among other things. A lot of influences are checked: driver initiated dissimilar crank undulations, but also misfire, defective catalytic converters, problems with the secondary air injection, exhaust gas recirculation and even the sensors themselves. This analysis takes time, the signals must be observed by the control device for a certain time. Only then the control device may provide clearance for the single areas. It must know how to distinguish normal and not normal sensor signals. If a sporadic mistake appears, by the way, during this time, it is not recognised by the control device.