Patent ID: 8286417

Claim:
A method for diagnosing an exhaust gas treatment system for reduction of nitrogen oxides in an exhaust gas train of an internal combustion engine by means of selective catalytic reduction, where a reducing agent is fed into exhaust gas via a metering device, with at least one NO x -sensitive sensor located downstream of an exhaust gas treatment device, comprising the following steps: defining at least one threshold value for NO x concentration for at least one operating point of the internal combustion engine; changing an operating state of the internal combustion engine from a usually prevailing normal operating state to a diagnostic operating state, a generated amount of NO x emissions in a diagnostic operating state being at least twice that of the normal operating state; determining the amount of reducing agent corresponding to the increased NO x emissions of the diagnostic operating state; controlling the metering device in such a way that an amount of reducing agent corresponding to the increased NO x emissions of the diagnostic operating state is fed into the exhaust gas; measuring a NO x concentration downstream of an exhaust gas treatment device; comparing the measured NO x concentration to the threshold value; deciding whether the exhaust gas treatment system is operating properly or malfunctioning, based on a comparison between the measured NO x concentration and the threshold value; and terminating the diagnostic operating state, with the internal combustion engine being switched back to the normal operating state or to a sensor diagnostic operating state for checking the proper functioning of the NO x sensitive sensor.