The Nextruder filament sensor does not detect filament properly (MK4, MK4S, CORE One)

Symptom

The printer says there is no filament when there is, or the other way round; it pauses the print for no reason, will not let you load, the sensor calibration fails in the wizard, or it shows «Fil. sensor troubleshooting» (#13114 on MK4, #26114 on MK4S, #31114 on CORE One), asking you to clean the sensor if it is unresponsive.

Likely causes, from most to least likely

1. Filament residue or dust built up around the sensor or its mechanism, causing false readings.
2. The sensor is out of calibration and its calibration has to be repeated.
3. On the MK4, whose sensor magnets come as separate parts for assembly, both magnets were fitted by mistake: one is for the assembly and the other is a spare.
4. A fault in the sensor mechanism (ball, magnet and spring). The CORE One has two sensors, one on the side and one in the print head, and the fault may be in either of them.

Checks to tell which one it is

• Check the state in Info → Sensor Info: with filament it must show inserted (INS) and without filament not inserted (NINS). On the CORE One check the print-head sensor and the side sensor separately.
• Insert and remove a piece of filament several times and watch whether the state always changes.
• If you assembled the MK4 yourself, check that there is only one magnet in the sensor assembly.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Clean the sensor with air

   Through the small hole at the top of the extruder, blow compressed air while repeatedly inserting and removing a piece of clean filament with a non-stringy tip, to dislodge the residue.

   Check: In Info → Sensor Info the state changes reliably when you insert and remove the filament.

   If that does not fix it: Recalibrate the sensor (next step).

2. Recalibrate the sensor

   Run Control → Calibrations & tests → Filament sensor calibration and follow the screen, which checks the sensor without filament and with filament.

   Check: The calibration finishes without errors and the sensor detects both states correctly.

   If that does not fix it: Check the sensor mechanism (next step).

3. Check the magnets and mechanism

   With the printer off and cold, partially disassemble the Nextruder following Prusa's guide to reach the ball, magnet and spring assembly, and clean it. On an MK4 you assembled yourself, leave only one magnet: the second one is a spare.

   Check: After reassembling and recalibrating, the sensor detects the filament correctly.

   If that does not fix it: While you sort it out you can turn the sensor off in the Settings menu (Filament Sensor on MK4 and MK4S; Filament Sensors on CORE One), knowing it will no longer detect filament run-out. If it keeps failing, contact Prusa support.

Prevention

• Clean the sensor with compressed air from time to time.
• Recalibrate the sensor after disassembling or repairing the extruder.

Still not fixed?

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Printers it applies to

Prusa MK4, MK4S and CORE One.

Official sources

This guide summarises the official documentation linked here:

• Nextruder filament sensor (CORE One, CORE One L, MK4/S, MK3.9/S, XL/+) · Prusa Research
• Fil. sensor troubleshooting #31114 (CORE One) #26114 (MK4S) #13114 (MK4) · Prusa Research
• LCD menu (MK4/S, MK3.9/S) · Prusa Research
• LCD menu (CORE One/+) · Prusa Research