Abstract: A method for determining the efficiency of an exhaust gas purification device of a diesel internal combustion engine for motor vehicles, having a first NOx sensor, arranged ahead of an oxidation catalyst and/or a particulate filter, and a second NOx sensor, arranged downstream of a reduction catalyst, and having a device for the metered supply of a reducing agent, wherein the method includes feeding the signals from the two NOx sensors are fed to a control unit, by means of which at least one feed quantity of the reducing agent is specified.