Control device and method for monitoring

Method for monitoring a motor vehicle by an on-board power supply system (4) provided power supply of a control unit (2) having a first input (12) and a second input (18), - wherein supplied to the first input (12) on-board network voltage (U) by means of an A / D converter (10) is monitored and a voltage value (14) is determined, - being supplied to the second input (18) via an electrical resistor (22), the vehicle electrical system voltage (U) directly or via a voltage regulator (8), and - determined from said A / D converter (10) voltage value (14) based on the input signal (26) at the second input (18) is verified.