Without additional measuring instruments, the on board diagnostics make it possible to compare the nominal- with the actual value. Thereby, not only are the exhaust gas values constantly controlled, but other engine functions as well. In case of disturbances, e.g., in the ignition system, the control device can interrupt and even cut off the fuel supply to individual cylinders. During the exhaust gas emission test, the garage tester is now only used, if it is equipped with an additional scanner
function or if a certain test is enforced by the readiness-code. In this case, only the individual areas are read out.
The examination of the exhaust gas values during the operational mode does not take place directly by checking the four exhaust gas values, but by the more exact analysis of the signals from, among others, the pre- and post catalyser sensor, the RPM sensor, The knocking sensor, and the acceleration transducer (ESP). The irregular crankshaft motion, possibly caused by the driver, misfiring, defective catalytic converters, problems with the secondary air injection, exhaust gas recirculation and even the sensors themselves are checked. For this to occur, the signals from the control device must be observed for a certain period. Only then can the control device confirm that the individual areas are error-free. Every single time it has to learn to distinguish between normal- and abnormal sensor signals. Should, in this period, a sporadic error occur, it will not be recognised by the control device.
In the above picture, a brand-name scanner can be seen, here one can be certain that a connection to the vehicle exists and that all the attainable values are also displayed. With testing devices, which are to be used with several brand-name vehicles, the depth of penetration depends on the amount of data that the manufacturer has released. In the meantime, there is an EU-wide obligation. Of course, a lot depends on the manufacturer of the testing device and also how consciencious the sales rep. is when it comes to installing updates.
Once a connection has been established, one starts in the 1st mode. Here, apart from the measured values, also the permitted tolerances and the possible substitute values, should these tolerances be exceeded, are most comprehensively displayed. In the 2nd mode conclusions are presented, which show the correlation of the error occurrence. The 3rd mode contains errors which have been stored in the past and the 7th mode shows short-term or sporadically appearing errors.