Abstract: The present invention relates to a method for checking or determining the installation air slot between an active sensor and an encoder, wherein the maximum size of the air slot (maximum air slot) which still ensures trouble-free operation of the sensor depends on the level of the supply voltage of the sensor at least within a range of said supply voltage, and which is more particularly characterized by the following method steps: reducing the supply voltage of the sensor to at least one value by which the maximum air slot is each time decreased by a desired test measure, and comparing the at least one test measure with a desired difference between the maximum air slot and the existing air slot by sensing a variation of the sensor output signal by the reduction of the supply voltage, as well as evaluating the comparison result by signaling and/or storing. A corresponding device is also described.