Filament Sensor - Quick Fix  

Filament Sensor - Quick Fix

My filament sensor suddenly stopped working (filament would not auto-load), so I took a look at the Prusa Knowledge Base for a solution. There was a good write-p there, but it involved extensive dismantling of the print head assembly.  It also mentioned that debris in the sensor can be one cause of the problem.

I decided to try a less invasive approach first: First I unloaded the filament and let then machine cool down. Then I removed the cover over the filament sensor, and then removed the screw holding the sensor in place. Without disconnecting the sensor I partially removed it. I next removed the screw holding the extruder idler door. Using a can of compressed air, I blew air into the sensor opening on top, and then into the opening of the idler mechanism. I repeated this twice, and noticed a small bit of plastic debris blow out.

I reassembled everything and tested the sensor - problem solved!  The whole procedure took about 5 minutes. Definitely worth trying first if you have filament sensor problems.