Method for diagnosing a NOx readings recorder which acquires a NOx concentration in an exhaust gas tract of an internal combustion engine and comprises two measuring chambers (110, 120), wherein the exhaust gas to be measured is supplied to the first measuring chamber (110) and an oxygen concentration is set by means of a first oxygen ion pump current (IP1), wherein the second measuring chamber (120) is connected to said first measuring chamber (110) and wherein both measuring chambers are disposed in a solid electrolyte, the oxygen content in the second measuring chamber (120) is determined; the oxygen content is additionally determined by a separate device; the two values characterizing the oxygen concentration are compared and a defective sensor is then suggested if the oxygen concentration value determined in the second measuring chamber (120) deviates from the oxygen concentration value determined by the separate sensor device by a predeterminable magnitude.