to a temperature sensor 30 the control unit 26 opens and closes the switches depending on the user commands heater on or off desired temperature and the temperature sensed by the temperature sensor 30 advantageously the control unit implements a pulse width modulation scheme in order to achieve the desired heating power or temperature the heating element 12 furthermore serves as an antenna electrode of a capacitive sensor 32 the capacitive sensor comprises an oscillator 34 which applies an oscillating ac voltage to the heating element 12 an alternating current sensor 36 measures the alternating current that flows into the heating element 12 the alternating current depends on the complex impedance z x represented as a resistance and a capacitance between the heating element 12 and ground a common mode choke 38 is arranged in the heating circuit in order to inhibit the flow of the alternating current to ground via the heating current source 18 the alternating current sensed by the alternating current sensor 36 is evaluated by a microcontroller 40 since the alternating current depends on the complex impedance between the heating element and ground a conductive object or a person in the vicinity of the heating element 12 can be recognized the capacitive sensor further comprises a diagnostic circuit for detecting in particular a short circuit or a circuit interruption between the first 14 and the second node 16 the diagnostic circuit comprises a heating current sensor 42 arranged between the first node 14 and the second node 16 in series with the heating element 12 the heating current sensor 42 is configured to measure the heating current that the heating current source 18 drives across the heating element 12.