The filament sensor gives false alarms or doesn't detect runout

Symptom

The printer pauses saying there's no filament (or that it's stuck) when there is, asks you to unload and reload for no reason, or autoload doesn't work. Or the opposite: the filament runs out and it keeps printing in the air.

Likely causes, from most to least likely

1. Filament residue or dust inside the sensor or its mechanism.
2. A sensor out of adjustment or not calibrated, on printers that calibrate it.
3. A mechanism that doesn't move freely: overtightened screws or badly assembled parts around the sensor lever.
4. A loose or wrongly oriented connector, or a damaged cable.
5. Firmware or configuration that doesn't match the installed sensor or extruder.
6. On sensors that monitor movement, the filament stops advancing (it drags or jams before the extruder) and the sensor takes it as runout.

Checks to tell which one it is

• Check the sensor state while inserting and removing a piece of filament: it should change reliably. In Klipper, with QUERY_FILAMENT_SENSOR SENSOR=name; on many printers, from a sensor info menu.
• If the state changes by itself without touching anything or never changes, the problem is in the sensor, its mechanism or its cable.
• If the state is correct but the printer still pauses, check whether the filament drags or jams before the extruder.
• If it started right after a firmware update or after changing extruder parts, check that the firmware matches your sensor and extruder version.

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 the printer cold, blow compressed air into the sensor to remove dust and debris. On some print heads this is done through a hole on top while inserting and removing a piece of filament with a clean tip several times.

   Check: The sensor state changes reliably when inserting and removing filament.

   If that does not fix it: Recalibrate the sensor.

2. Recalibrate the sensor

   If your printer has a filament sensor calibration, run it again following your model's procedure.

   Check: No more false alarms.

   If that does not fix it: Check the connector, cable and mechanism.

3. Check the connector, cable and mechanism

   With the printer off and unplugged, check that the sensor connector is fully seated and the right way round, and that the lever or mechanism moves freely; don't overtighten the screws around it.

   Check: The sensor reliably detects filament present and absent.

   If that does not fix it: Check the firmware and configuration.

4. Check firmware and configuration

   Make sure you have the right firmware for your extruder and sensor version. In Klipper, review the sensor section in printer.cfg; in Marlin, M412 with no parameters reports whether detection is enabled.

   Check: Detection is enabled and the sensor works when tested by hand.

   If that does not fix it: Finish the print with the sensor disabled and replace it afterwards.

5. If you must finish a print, disable it temporarily

   Disable the sensor only while you fix it: in Marlin, M412 S0 (M412 S1 turns it back on); in Klipper, SET_FILAMENT_SENSOR SENSOR=name ENABLE=0; on other printers, from the menu. Watch the filament yourself: without the sensor, the printer won't stop if it runs out.

   Check: The print finishes without false pauses.

   If that does not fix it: Repair or replace the sensor before enabling it again.

6. If it doesn't detect runout

   Check that detection is enabled and test the sensor by hand. In Marlin, M412 D sets how much filament keeps extruding after runout is detected, based on the distance from the sensor to the extruder gears.

   Check: When you remove the filament, the printer pauses or asks for a change.

   If that does not fix it: Replace the sensor or contact your manufacturer's support.

Prevention

• Clean filament residue from the sensor during routine maintenance.
• Don't leave it disabled: without it, the printer won't warn you when the filament runs out.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Fil. sensor troubleshooting · Prusa Research
• IR Filament Sensor Troubleshooting (MK2.5S, MK3S) · Prusa Research
• G-Codes (QUERY_FILAMENT_SENSOR, SET_FILAMENT_SENSOR) · Klipper
• M412 - Filament Runout · Marlin