the invention concerns a method and a device for testing the function capacity of an no oxidation catalyst which is used to reduce nitrous oxides contained in the exhaust gas flow of an internal combustion engine operated with air surplus . in the exhaust gas flow which is supplied to the no oxidation catalyst , a change is made in the concentration of a reducing agent and the resulting change in no x concentration in the exhaust gas flow within the no oxidation catalyst and / or downstream after the no oxidation catalyst is determined and used to test its function capacity .