A method for operating a sensor which is configured for checking value documents and in which at least one operating mode (B1, B2) is provided for checking the value documents, wherein the sensor, for testing its functionality, carries out a self-test in which it is provided that the sensor reacts to at least one malfunction (F1, F2, F3, F4) which is ascertained during the self-test and that would hinder the check of the value documents in the provided operating mode (B1, B2) by the sensor employing for checking the value documents, instead of the provided operating mode (B1, B2), a modified operating mode (B1', B2') in which at least one other measured value of the sensor is employed for checking the value documents than is determined in the operating mode (B1, B2) provided for checking the value documents, characterized in that - the sensor is an optical sensor which can detect the light emanating from the value documents at several wavelengths, and that the optical sensor for checking the value documents in the modified operating mode (B1', B2') - employs at least one measured value which is detected at another wavelength than the measured values which are determined in the provided operating mode (B1, B2) for checking the value documents, or - employs, instead of a measured value affected by the malfunction, a derived measured value which is derived from measured values which are detected spectrally adjacent to the measured value affected by the malfunction, - or that the sensor has several measuring tracks (L1-L12) transverse to a transport direction of the value documents along which the value documents are transported past the sensor for their checking, and that the sensor for checking the value documents in the modified operating mode (B1', B2'), in the case of a malfunction of one of the measuring tracks, - employs for checking the value documents, instead of the measured value of a measuring track affected by the malfunction, the measured value of another measuring track which is adjacent to the measuring track affected by the malfunction, or - employs, instead of the measured value of the measuring track affected by the malfunction, a derived measured value which is derived from the measured values of measuring tracks which are adjacent to the measuring track affected by the malfunction. - the sensor is an optical sensor which can detect the light emanating from the value documents at several wavelengths, and that the optical sensor for checking the value documents in the modified operating mode (B1', B2') - employs at least one measured value which is detected at another wavelength than the measured values which are determined in the provided operating mode (B1, B2) for checking the value documents, or - employs, instead of a measured value affected by the malfunction, a derived measured value which is derived from measured values which are detected spectrally adjacent to the measured value affected by the malfunction, - employs at least one measured value which is detected at another wavelength than the measured values which are determined in the provided operating mode (B1, B2) for checking the value documents, or - employs, instead of a measured value affected by the malfunction, a derived measured value which is derived from measured values which are detected spectrally adjacent to the measured value affected by the malfunction, - or that the sensor has several measuring tracks (L1-L12) transverse to a transport direction of the value documents along which the value documents are transported past the sensor for their checking, and that the sensor for checking the value documents in the modified operating mode (B1', B2'), in the case of a malfunction of one of the measuring tracks, - employs for checking the value documents, instead of the measured value of a measuring track affected by the malfunction, the measured value of another measuring track which is adjacent to the measuring track affected by the malfunction, or - employs, instead of the measured value of the measuring track affected by the malfunction, a derived measured value which is derived from the measured values of measuring tracks which are adjacent to the measuring track affected by the malfunction. - employs for checking the value documents, instead of the measured value of a measuring track affected by the malfunction, the measured value of another measuring track which is adjacent to the measuring track affected by the malfunction, or - employs, instead of the measured value of the measuring track affected by the malfunction, a derived measured value which is derived from the measured values of measuring tracks which are adjacent to the measuring track affected by the malfunction.