A monitoring device for at least one auxiliary assembly which influences the exhaust-gas quality of an internal combustion engine and consumes a resource in order to attain a desired quality of exhaust gas, includes at least one sensor for measuring a variable related to the resource, and an arithmetic unit for estimating the current quantity of the resource on the basis of the variable measured by the at least one sensor, and which is designed to calculate and indicate an operating capacity of the internal combustion engine that remains before the resource is depleted. Potential resources are the absorbance capacity of a particulate filter and/or the quantity of an ammonifier that is available.