An aerodynamically effective device for a motor vehicle has an air guide element that can be moved between a position of rest and at least one operating position, a drive unit for moving the air guide element, and a control unit. The control unit is adapted to instruct the drive unit to move the air guide element from the position of rest into the operating position, on reaching a first vehicle speed threshold value. Furthermore, the control unit is adapted to carry out a function test of the drive unit below the first vehicle speed threshold value.