Filament sensor (MK4, MK3.9)

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The filament sensor equipped on the Original Prusa MK4 and Original Prusa MK3.9 detects the presence of a filament and filament run-out. The sensor is a Hall sensor, triggered by a mechanism composed of a spring, a magnet, and a ball. 

During usage, it is possible that filament residue might build up around the sensor or its mechanism, possibly causing false readings and thus malfunctioning. 

How to clean the filament sensor 

1) Compressed air

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.

2) Filament tip insertion

In case you don't have compressed air at hand, a similar procedure to the one above can be attempted.

Prepare a bit of filament with a non-stringy tip. Repeatedly insert and remove the filament from the extruder. It will be necessary to insert and remove the bit of filament several times to achieve good cleaning, and the technique might not always be effective. 

3) Removing the sensor

If the previous techniques do not work, the filament sensor can be removed to be manually cleaned. This procedure requires partial disassembly of the E-axis. 

1. Unload the filament, then wait for the printer to fully cool down. After that, turn the printer off.
2. Remove the PG-case, together with the PG-ring, gearbox, and main plate. Remove all of these parts in one piece.

3. Remove the Prusa-ball-holder using pliers.

4. Carefully remove the ball-magnet-spring combo from the Prusa-ball-holder and clean any debris out of it. Clean also the slot on the heatsink.

5. Reassemble the Prusa-ball-holder in the following order: steel ball, magnet, and spring.

6. Reassemble the block with PG-case, PG-ring, gearbox, and main plate.