Fil. sensor troubleshooting

What happened? 

The filament sensor on the print head of the Original Prusa MK4/S, MK3.9/S, MK3.5/S, equipped with an MMU3 upgrade kit, can detect filament runout, but also whether the filament is moving regularly or not. 

In case of a detected blockage close to the filament sensor, the printer shows the message "Fil. Sensor Troubleshooting. If the filament sensor is unresponsive, clean it from debris".

Error name: Filament sensor stuck help MMU

Error code: #26114 (MK4S) #13114 (MK4) #27114 (MK3.9S) #21114 (MK3.9) #28114 (MK3.5S) #23114 (MK3.5)

How to fix it? 

Clean the filament sensor (MK4/S, MK3.9/S)

At the top of the extruder is a small hole. Insert the tip of a compressed air can in the hole. With your free hand, prepare a bit of filament with a non-stringy tip. Repeatedly insert and remove the filament from the extruder, while blowing the compressed air. This will dislodge the build-up debris. 

Access the filament sensor (MK3.5/S)

On the MK3.5/S print head, on the top of the chimney assembly, is a cover for the filament sensor. use a 1.5mm Allen key to loosen one M2x8 screw, remove the cover and access the filament sensor. 

Clean debris from the sensor if there is any. When reinstalling the cover, do not overtighten the M2x8 screw. 

Recalibrate the filament sensor

A filament sensor miscalibration can be behind the issue. Recalibrate it, following the steps specific for your printer model. 

• MK4/S, MK3.9/S
• MK3.5/S