Prusa error 13804 (Filament Not Detected) appears on the MK4 and its equivalents are #31804 on the CORE One, #35804 on the CORE One L, #26804 on the MK4S, #27804 on the MK3.9S, #28804 on the MK3.5S, #21804 on the MK3.9, #23804 on the MK3.5, and #17804 on the XL. The filament runout sensor is enabled but does not detect filament at the sensor position — either the printer is about to run out or filament is not loaded correctly past the sensor.

What the Error Means

The filament sensor sits in the extruder feed path and must have filament physically passing through it to register as present. This error fires when the printer checks the sensor at print start or during a print and finds the sensor is in the “no filament” state. This could mean the spool has actually run out, the filament loaded from above the extruder has not been threaded far enough to reach the sensor, the filament has broken inside the PTFE tube above the extruder, or the sensor itself has developed a fault that causes it to read “absent” even when filament is present.

Step-by-Step Fix

1. Confirm filament is on the spool and feeding freely. Check that the spool is not empty and is rotating freely. Tangles or spool holder friction can stop filament from advancing even when material remains on the spool.
2. Use the Load Filament function from the LCD menu. Navigate to LCD Menu → Filament → Load Filament and let the printer motor pull the filament down through the PTFE tube and into the extruder. This ensures the filament tip passes through the sensor position.
3. Check for filament breaks inside the PTFE tube. If filament snapped during unloading, a fragment may be stuck above the extruder without reaching the sensor. Remove the PTFE tube connection at the top of the Nextruder and push the fragment through manually.
4. Inspect the sensor if filament is definitely loaded. If you can see filament entering the extruder but the sensor still reads “not detected,” the sensor may be dirty or faulty. Check for debris around the sensor trigger area.
5. Temporarily disable the sensor if needed to proceed with a print. Navigate to LCD Menu → Settings → Filament Sensor and disable it to bypass the check. Re-enable after resolving the underlying cause.

Parts Required

• No replacement parts are typically required for this fix — basic tools only
• Replacement filament sensor (if the sensor fails to detect confirmed-present filament)

If filament is correctly loaded and the sensor still does not detect it, contact Prusa Support at help.prusa3d.com for sensor replacement guidance.

Frequently asked questions

What does Prusa 13804 mean?

Prusa error 13804 (Filament Not Detected) appears on the MK4 and its equivalents are #31804 on the CORE One, #35804 on the CORE One L, #26804 on the MK4S, #27804 on the MK3.9S, #28804 on the MK3.5S, #21804 on the MK3.9, #23804 on the MK3.5, and #17804 on the XL. The filament runout sensor is enabled but does not detect filament at the sensor position — either the printer is about to run out or filament is not loaded correctly past the sensor.

How do I fix Prusa 13804?

Prusa error 13804 means the filament sensor cannot detect filament in the extruder. Learn how to load filament correctly and troubleshoot a non-detecting sensor.