Sensor and method for operating the sensor

A sensor for checking different features of value documents is arranged to carry out a self-test for testing its functionality. Before the sensor carries out its self-test, at least one of the features that is to be checked by the sensor is selected. It is provided in the self-test that the sensor rates a malfunction ascertained in the self-test differently and reacts to the ascertained malfunction differently in dependence on the selected feature. While a malfunction of the sensor has hitherto always led to a failure of the sensor, the sensor, despite ascertained malfunctions, can nevertheless carry out the check of certain features. It is achieved that the number of function failures of the sensor is reduced through the different reactions of the sensor to the ascertained malfunction in dependence on the feature.