MK3S keeps prompting filament change

Updated 4 months ago ​by Martin Lexa

With the Original Prusa MK3S (and the Original Prusa MK2.5S upgrade), we are introducing a completely reworked extruder with a new filament sensing system. It is a hybrid solution, combining an IR laser sensor triggered by a tiny mechanical lever. This system is not influenced by the optical properties of the filament and is immune to mechanical wear.

What does it look like

This guide is related to the situation when the filament autoload is working (or even if it would trigger only sometimes), yet the printer keeps asking you at the beginning of a print to unload the filament and then to load it back in, in other words the printer keeps prompting filament change. This means that the filament sensor by itself is working, but not as it should.

In case the sensor is not working at all, and it is shown as N/A under Settings in the LCD Menu, you have to double check its wiring (on both ends, but especially its connection to the RAMBo board) and see if the sensor itself is not damaged.

In general, the Filament sensor is not vital for the printer, meaning you can always turn it off in the LCD Menu -> Settings -> Fil. sensor (toggle between [on] and [off]) and continue using the printer.
Also, make sure you have not installed the sensor upside down.

How to fix it

First of all, make sure you are running on firmware version 3.5.3 or higher. Always make sure you have the latest firmware version for your printer model.

  1. The first step is to toggle the Filament autoload off. Do it in the LCD Menu -> Settings -> F. autoload. (but the Fil. sensor must be still [on]!)
  2. Now we need to get to the filament sensor mechanism. Before you start, make sure the printer is cold and that none of the fans is spinning.
    1. Take the Print fan off by unscrewing both M3x20 screws holding it on the Extruder.
    Do not lose the nuts for these screws ;)You can insert the fan in between the belts on the left side, it should fit there just fine.
    1. Unscrew 2 out of 4 screws of the Noctua Hotend fan that are the closest to the original position of the Print fan. One of them is M3x18 (or M3x20 if you decided to use it instead) and the other one is th top-right M3x14.
    2. Unscrew both long M3x40 screws located below the Extruder motor and take the Extruder cover off.
    3. Unscrew the M3x40 screw holding the Extruder idler door as well as the other M3x40 screw holding the upper part of the assembly together.
    4. Take the Extruder motor off.
    Be careful not to damage anything, especially not the Filament sensor.
    1. Congratulations, you now have a full access to the hotend and to the filament sensor mechanism .
  3. Now you need to go to the LCD Menu -> Support -> Sensor info
    1. Here you can see the status of all 3 sensors we are using on our printers: the PINDA, the FINDA and the IR sensor (which is one of the designations of the new filament sensor).
    2. See how the IR sensor status changes from 0 to 1 as you are inserting a piece of filament inside the opened extruder. Also, observe how the filament sensor lever moves.
    3. To be sure, blow some canned air (or just blow with your mouth) in the general direction of the sensor to get rid of any potential dust particles.
The IR sensor value should be a stable 0 when there is no filament and a stable 1 as long as the filament is present.

Color of the plastic parts

Please note that the FS-lever has to be printed in black PETG. The color of the extruder plastic parts can be different, but the tiny FS-lever has to be in black.

Make sure the lever is moving correctly

All in all, the lever has to move freely and get pushed by the steel ball in a regular manner.

  • Make sure the steel ball is in its housing.
  • Make sure the you did not forget to put in the M3x18 screw that serves as a shaft for the lever movement. This M3x18 screw also has to be tightened just right. Overtightening it makes the lever not move or move insufficiently. Having it too loose might make the lever move in other directions than intended.
  • Also make sure the magnets have the correct orientation. They need to be repelling one another.

After fixing this issue, assemble the extruder following the manual you can also turn the Filament autoload back on. Re-do the Z calibration, it should not be necessary to re-do the entire XYZ Calibration, unless you have by mistake touched/moved with the PINDA.


Find out more troubleshooting tips here: Community Forum | Assembly Manuals | Youtube channel |

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