1. A method for checking value documents using an optical sensor configured to detect light emanating from the value documents, a plurality of variables being measurable by the optical sensor, one of the variables being a preferred variable to measure for checking the value documents, the method comprising: performing a self-test of the optical sensor; when the self-test indicates a malfunction of the optical sensor that would hinder an ability of the optical sensor to check value documents, using the preferred variable, using a second one of the variables to check the value documents by measuring, by the optical sensor, the second variable as the value documents pass by the optical sensor; otherwise using the preferred variable to check the value documents by measuring, by the optical sensor, the preferred variable as the value documents pass by the sensor.