In a diagnosis method of a control valve, position data representing a position of a control valve, and pressure data representing a pressure difference over a valve actuator, and optionally travel direction of the control valve, is measured during online operation of the control valve. The position data and the pressure difference data are processed to contain data around starting points of a plurality of individual travel movements of the control valve during normal online operation of the control valve. Finally, a valve signature graph of the control valve is determined based on the processed position and pressure difference data, collected at a plurality of points along the travel range of the control valve during online operation of the control valve.