Why does my side filament sensor keep showing "filament present" even after I remove the filament? It still displays "ON" when the filament is out.  

It may be blocked, stuck or faulty.

First check the sensor wire connections in case they have become loose.

Blow some compressed air through the sensor to shift any blockage - even a whisp of stringing can be enough to hold it open.

I have experimented with pipe cleaners after using slightly sticky filament - this is *NOT* Prusa recommended but I had some to hand, twisted wire with soft bristles intended to remove tar from inside smoker's apparatus, you need the thinner version.  Add a drop of IPA and use it to clean through.

Repeatedly inserting and removing a stub of filament might jog a stuck sensor free.

Otherwise you will have to disassemble and investigate.

Cheerio,

RE:

Posted by: @chujian1979

Why does my side filament sensor keep showing "filament present" even after I remove the filament? It still displays "ON" when the filament is out.

In my kit build, the little magnet in the sensor's moving lever was the problem. The lever is 5 mm wide, but the magnet measures about 6 mm, so it sticks out a bit on the sides. If one lets it stick out too far on the upper side (towards the lid of the sensor housing), the lever will move freely while the housing is still open, but will get stuck once the lid is closed.

In my case, the lever could move just enough to let filament pass when I first inserted some, but then did not come back into its resting position when the filament was removed. 

You will have to take off the right side panel, disassemble the carrying handle & sensor, open the sensor housing and nudge the magnet into a well-centered position in the lever. When re-attaching the side panel, be sure that the cable to the filament sensor does not get stuck in its cable ties and forms a little loop -- it can easily get in the way of the Y linear bearing.