Abstract: Signals generated by a knock sensor are coupled to an electronic network for amplifying and processing the output signals of the knock sensor. The electronic network includes a device for charging a storage capacitor to a potential which is a function of the amplitude of the knock sensor output signal. The storage capacitor is coupled to the input of an amplifier via a normally closed first switch. A detector for sensing any disconnection of the knock sensor from the electronic system produces a control signal indicative of a disconnect condition. The control signal is used to open the normally closed first switch and to cause the input of the amplifier to be clamped to a reference potential which lies outside the signal range normally produced across the storage capacitor whereby the output of the output of the amplifier is placed at a predetermined level indicative of a disconnect condition.