Diagnostic Overview

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To cover aging effects in the emission related exhaust system, for parts like catalysts as well as pre- and post oxygen sensors, a fuel trim strategy is used. As a part of the fuel trim the post oxygen sensor signal is taken into account.

For shifting the post oxygen sensor signal on a certain voltage level, the closed loop controller is used. The expected rear oxygen sensor signal is compared to the current signal. As long as the expected value is not reached, the closed loop controller is shifting the system by a certain amount of proportional or integral share. The outcome is a corrected lambda value (fuel trim value is added to the base fuel calculation).

In case it is not possible to shift the system on the expected level, the fuel trim diagnostic which cover validation of the proportional/integral value of the fuel trim. The fuel trim data will be compare with a application threshold. As soon as the threshold is exceeded a malfunction code will be set for the specific criteria.

It is to mention that before a malfunction for trim control is set, a validation of front and post oxygen sensor is performed through introducing a rich/lean shift. Please see also within the description of linear oxygen sensor monitoring.