30 in the event of an improper position of the first sheet guide module 25 the coupling elements 31 32 are necessarily at least partly separated from one another so that the mutually associated electrical contacts 33 35 and or the mutually associated electrical contacts 34 36 are separated from one another and consequently the current flow in the electrical coupling 30 is interrupted between the signal lines 37 38 this interruption to the current flow is a signal that indicates from the machine control system 42 that the first sheet guide module 25 is located out of its intended operating position required for printing operation the signal interpretation leads to the machine control system 42 switching over the sheet fed printing press 1 from a first operating mode into a second operating mode the second operating mode is distinguished by the fact that the machine control system 42 firstly by an optical or acoustic warning signal points out the hazardous situation to the operating personnel and secondly at the same time prevents specific movements of the sheet fed printing press 1 for example the warning signal can appear on a display of a non illustrated central main control desk of the sheet fed printing press 1 or can be a flashing light or a flashing key on the printing unit 2 a 2 b affected by the damage or the removal of the sheet guide module which movements of the sheet fed printing press 1 that are prevented by the machine control system 42 in the second operating mode depends on whether the protective cover 17 is open or closed if the sensor 19 signals to the machine control system 42 that the protective cover 17 is closed then none of the operating.