A device for adjusting a parameter includes a selector that is actuated by an operator. A coder is driven in rotation by the actuation of the selector and emits a signal representative of the rotation. A first detector detects an actuation of the selector, and a second detector detects the emission of the signal indicating the change of the selected value. A comparitor, linked to the first and second detectors, determines a malfunction when only one of the first and second detectors makes a detection.