To form the image on the cylinder, this printer uses a marking unit consisting of four print heads mounted 2 on each carriage (the upper and lower carriages). In the process of printing and cleaning, the carriage are driven by a series of gears and motor system. At your printer is a problem when moving the lower carriage. It may be a problem with a dirty sensor or with the carriage drive system.
The part no. for the carriage drive train is 041K06600.
It is not really that simple You normally need a tech. But if you want try my indications below.
Open the front door, remove the front cover (6 screws), remove the cleaning unit handle. After removing the front cover you will see on the left side the Marking Unit. Remove the 2 screws on the bottom of the unit and pull it from the handle to you. On the next image you will see noted with 8 the upper and lower carriage sensors which you can clean them.
Also you can move by hand with the large gears (9 and 10) to see if there is no blockadge. To unlock upper or lower carriage move up or down the knob on the left side of the drive train.If this will not solve the issue it could be also the Marking Unit Driver PWB (board) defective.