Assembly and series of steps to determine the operating efficiency of an automotive exhaust selective catalyst reduction unit

An automotive exhaust system has a diagnostic unit determining the condition of the selective catalyst reduction unit. In a series of stages, in the first step the diagnostic system defines the threshold NOx concentration under one or more defined operating conditions. In subsequent steps the unit then shifts the engine's operational mode into an operational diagnostic condition in which more NOx is produced than in the normal mode. The dosing device is subsequently regulated to adjust the quantity of reduction agent corresponding to the change in NOx emissions. The concentration of NOx is measured downstream from the post-treatment station and compared with the threshold value. A decision is then made as to the absence of error in operation of the exhaust gas treatment system based on a comparison of the measured concentration of NOx with the threshold vale, followed by termination of the diagnostic stage. also claimed is a commensurate automotive diagnostic assembly.