A controlling unit 124 and a method of correcting an actual limit value 70, 92 of a sensor signal is disclosed, wherein above or below the limit value a specific state of the sensor is recognized, the method comprises the steps of determining a new limit value of the sensor 66, 87, determining a stability criteria 50 to decide whether the actual limit value 70,92 is corrected with said new limit value 66, 87 and approaching the actual limit value 70,92 to the new limit value 66,87 when the conditions are fulfilled to correct the actual limit value 70,92 for said sensor.