A device for activating an electropneumatic valve of a pressure medium-operated position controller, in which the valve is activated by a manipulated variable. To achieve a blocking fail-safe behavior in case of a failure of a pressure medium, the device includes means for detecting the current position of a booster stage downstream of the valve and assigning the manipulated variable based on the detected position, and means for measuring a first variable to be influenced by the position controller. The device includes an evaluation unit configured to determine an expected directional reaction of the first variable to the assigned manipulated variable, compare the expected reaction with the measured reaction, determine a pressure medium failure as a fault situation if the expected reaction does not coincide with the measured reaction, and assign, as a new manipulated variable for the valve, an emergency signal generating a blocking fail-safe behavior of the valve.