To locate the drop detector, just power down the plotter, open the window and the door on the right hand side. Pull the printhead cleaners towards you until it stops, the carriage should then be unlocked. Slide carriage over to the left. Look for a black box just to the right of a white drive roller gear. On that black box there is a rectangular opening, this is the drop detect sensor.
This is the part you need to check for any paper fibres stuck in it. Clean around the top of the opening and around the inner part of the rectangular opening with a damp coffee filter, be careful.
When finished, just leave the carriage and the printhead cleaners where they are, close the right hand door and top cover, then power the plotter back on.
Then you will need to perform the service station calibration. Go to setup menu, then to utilities, press enter, then UP and ENTER at the same time, that will bring you to the service station calibration, follow the steps.
Hope this helps you out.