a sensor signal from a sensor which measures vibration of an internal combustion engine is filtered by a filter to provide characteristic frequency components for a normal combustion . when the filter signal remains below a reference value , a fault signal is output to indicate a faulty combustion . the filter may be switchable band - pass filters whose output signals are measured for knocking of the motor or for normal combustion . a programmed control unit evaluates the output signals from the filter within a characteristic time segment .