A method and apparatus for detecting the faulty working of spinning units of an open-end spinning machine by monitoring the faults in yarn as it is fed from the spinning machine. An electrical signal is produced by a sensor indicating faults appearing in the yarn. Another signal of predetermined frequency corresponding to the speed of delivery of the yarn from the spinning machines is generated. The electrical signal produced by the sensor is electrically multiplied by the signal of predetermined frequency for producing a differential signal. This differential signal is filtered, shaped and integrated and subsequently compared with a preset threshold value for producing a signal indicating when the occurrence of faults exceed a predetermined threshold. This signal can be used for stopping the spinning unit.