A method for verifying the plausibility of a measuring device for determining a quality of an aqueous urea solution in a reservoir of an SCR catalyst system comprises determining whether a filling of the reservoir has taken place, determining the quality of the aqueous urea solution in the reservoir and detecting whether an error of the measuring device is present or whether the reservoir was filled with an aqueous urea solution, the quality of which deviates from a default value consisting of a filling status of the reservoir and the quality of the aqueous urea solution in said reservoir.