Disclosed is a plug-in connector module having at least one optical waveguide and at least one optical sensor, preferably a photodiode, which is arranged near the optical waveguide. The optical sensor can reliably detect malfunction of the plug-in connector module in good time. Also disclosed is a method for detecting signal losses during signal transmission in an optical plug-in connector module, in which an optical signal is guided through at least one optical waveguide of the plug-in connector module, and the optical signal is scattered in the event of a structural defect of the optical waveguide or dirt on the front surface of the optical waveguide, the scattered light reaching a photodiode and a current or a voltage being produced thereby on the photodiode. If a threshold value of the current or the voltage on the photodiode is exceeded, an interference signal is generated by an electronic evaluation system.