Diagnosis method for an exhaust gas post-treatment system

x x x x x x x The invention relates to a diagnosis method for an exhaust gas post-treatment system for the reduction of nitric oxides in the exhaust gas train of an internal combustion engine by means of selective catalytic reduction. The following steps are provided for easy recognition of errors: definition of at least one threshold value for the Noconcentration for at least one engine operation point; shifting the operational mode of the internal combustion engine into an operational state of diagnosis, wherein in the operational state of diagnosis substantially more Noemissions are produced than in the normal operational mode; determining an amount of reducing agents corresponding to the increased amount of Noemisions in the operational state of diagnosis; controlling a dosing device in order to introduce the amount of reducing agents, corresponding to the increased amount of NOemissions during the operational state of diagnosis, into the wase gas; measuring the concentration of NOdownstream from the post-treatment device; comparing the measured concentration of NOto the threshold value; taking a decision as to the erroneous or error-free operation of the waste gas post-treatment system based on comparison of the measured concentration of NOto the threshold value; terminating the operational state of diagnosis.