MK3S+: the IR filament sensor fails (asks for filament change, no autoload or no run-out detection)

Symptom

On the MK3S+ autoload does not start when you insert filament, the printer does not pause when the filament runs out, or at the start of every print it asks you to unload and reload the filament even though it is loaded correctly.

Likely causes, from most to least likely

1. Dust or filament strings near the sensor or holding its lever back.
2. The G-code was sliced with a multi-material profile, which is why it asks for filament changes at the start.
3. The firmware is not the S/S+ version this sensor needs, and calibration fails because of the differences in the extruder parts.
4. The lever does not move freely: the M3x18 screw acting as its shaft is too tight or too loose, the steel ball is out of its housing or the magnets face the wrong way (they must repel each other).
5. Overtightened screws near the sensor: the M3x40 that goes into the extruder motor from the back, the M2x8 holding the sensor or the M3x10 of the FS-cover plate.
6. The sensor cable is badly connected to the board.

Checks to tell which one it is

• Go to Support → Sensor info: the sensor value (Fil. sensor; IR sensor in older firmware) must be a stable 0 without filament and a stable 1 with filament.
• Watch whether the lever moves freely and returns by itself when you insert and remove the filament.
• Try printing one of the sample G-codes bundled with PrusaSlicer: if it does not ask for a filament change, the problem is the profile you sliced with.
• Check that the installed firmware is the S/S+ version for this printer.

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. Remove the dust

   Blow compressed air towards the sensor to remove dust and filament bits.

   Check: The sensor responds the same way every time you insert and remove the filament.

   If that does not fix it: Check the slicing profile (next step).

2. Slicing profile

   If it asks for a filament change at the start, check that you did not slice with a multi-material profile and try a sample G-code from PrusaSlicer.

   Check: With the right profile the print starts without asking for a filament change.

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

3. Correct firmware

   Install the MK3S/MK3S+ firmware in its S/S+ version: with other firmware the sensor does not calibrate properly.

   Check: The sensor value changes from 0 to 1 when you insert filament.

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

4. Lever, ball and magnets

   With the printer off, check that the steel ball is in its housing, that the M3x18 screw acting as the lever shaft is fitted and tightened just right, that the magnets repel each other and that no filament string holds the lever back. If the M3x40 motor screw, the M2x8 sensor screw or the M3x10 cover screw are very tight, loosen them slightly.

   Check: The lever moves freely and returns by itself; the value changes from 0 to 1 steadily.

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

5. Sensor cable

   With the printer off and unplugged, check the sensor connector on the board: according to Prusa, on the MK3S+ it goes on the bottom row of pins, covering all of them, with the red wire on the right.

   Check: Sensor info shows a stable 0 and 1.

   If that does not fix it: Meanwhile you can turn the sensor off in Settings → Fil. sensor [OFF]; the printer works without it but will not detect run-out. If it still fails, contact Prusa support.

Prevention

• Turn autoload off (Settings → Autoload) before doing a cold pull or working on the extruder.
• When assembling or repairing the extruder, do not overtighten the screws near the sensor.
• Keep in mind that this sensor detects filament presence and run-out, not clogs.

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Printers it applies to

The Prusa MK3 family: MK3S+.

Official sources

This guide summarises the official documentation linked here:

• IR Filament Sensor Troubleshooting (MK2.5S, MK3S) · Prusa Research
• IR Filament Sensor (MK2.5S, MK3S/+, MK3.5/S) · Prusa Research
• MK3S keeps prompting filament change · Prusa Research