A method for diagnosing a position detector used in determining the position of a control means. The position detector includes a resistance track, and a first terminal in connection with its first end, and a second terminal in connection with second end of the resistance track, and a slide electrically connected to the resistance track. The slide is arranged to move in relation to the control by the effect of a position change. Additionally, the position detector includes a slide terminal in connection with the slide, an electricity supply, a voltage measurer and a signal processing unit for diagnosing measurement data. In the method, a supply voltage is arranged in the slide terminal, the output voltage of the first terminal is measured, the output voltage of the second terminal is measured, and at least a first and a second output voltage are arranged to the signal processing unit.