Passenger transport system with monitoring and marking device for characterizing defective step units

A method for modernizing an existing passenger transport system, wherein the passenger transport system has multiple step units and a comb plate, wherein the step units are to be moved one behind the other continuously along a predefined movement path during normal operation, and the comb plate is arranged at a fixed position relative to the predefined movement path,

• the method comprising:
• retrofitting the passenger transport system with a monitoring device and a marking device;
• the monitoring device configured to monitor an actual position of a currently monitored step unit relative to the predefined movement path and to detect a deviation of the actual position from the predefined movement path by more than an allowable tolerance value and activate the marking device in response thereto;
• wherein upon being activated, the marking device is configured to provide the currently monitored step unit and/or a step unit spatially correlating to the currently monitored step unit in a defined manner with a marking.