Abstract: A diagnostic method for catalytic converters includes the steps: repeated measurement of an actual temperature downstream from a catalytic converter volume, repeated calculation of a first model temperature on the basis of a first temperature model, and evaluation of a conversion capability of the catalytic converter volume on the basis of a first difference which is a function of the actual temperature and the first model temperature. The method is characterized in that a second model temperature is additionally calculated on the basis of a second temperature model, a second difference is formed which is a function of the second model temperature and the first model temperature, and the conversion capability is evaluated on the basis of a value which is a function of the first difference, this value being normalized to a value which is a function of the second difference.