A combustion engine has in the exhaust gas area a SCR-catalyzer with a NH3-filling level, a metering device for supplying the SCR-catalyzer with reactant for reducing nitrous gases, and a downstream NH3-sensitive NOx-sensor. One theoretical value for the NOx downstream of the SCR-catalyzer is compared to an actual NOx-value downstream of the SCR-catalyzer. A reasonability check includes determining a first value for one comparison parameter, which represents the state before a system intervention for adapting the NH3-filling level. A system intervention for adapting the NH3-filling level is carried out and a second value of the comparison parameter is detected. From the comparison of the first value and the second value the reasonability, the non-reasonability, or non-preciseness of the system intervention is assumed.