A switch is formed by a branch including a measurement resistance and a selectable one of switching resistances having different nominal values such that a switch position depends on which switching resistance is selected. The measurement resistance is connected through the selected switching resistance to a supply voltage whereby a voltage drop dependent on the switch position is across the measurement resistance. A controller identifies the switch position by comparison of a measured value (UM1) of the voltage drop with target voltage drops corresponding to switch positions, calculates a voltage drop (Uges) across the branch based on nominal values of the measurement and selected switching resistances and the supply voltage, forms a difference value (UDiff) between a measured value (UVcc) of the supply voltage and the branch voltage drop (Uges), and qualifies the identified switch position as faulty when the difference value (UDiff) exceeds a difference threshold value (UDiff-S).