Printer stopped extruding mid-print: what to check first

This page is a triage guide. It helps you work out which of the common causes stopped your print, then sends you to the guide that covers that cause in full. It also covers what Prusa and Bambu Lab printers do when their filament sensors see a runout, and how each maker says to resume. If extrusion was weak from the very first layer, start with our under-extrusion guide instead. If nothing came out for the first moves and printing then recovered, see our guide to no extrusion at the start of a print.

What did the printer do when it stopped?

Before you touch anything, note what state the printer is in. It narrows the causes quickly.

Paused, or still moving?

Bambu Lab's tangle detection page for the A1 series explains the gap: when filament tangles on the spool, "the filament detection sensor of the tool head is still triggered," so it "cannot trigger the filament-run-out reminder." The result is what Bambu calls "air printing": the printer carries on moving with no plastic coming out.

So a runout message means the sensor saw no filament. A printer that keeps moving silently, with filament still at the sensor, points away from an empty spool. Bambu's P2S page lists nozzle clogs, filament wear, irregular filament diameter and wrong temperature settings as causes of air printing, and its A1 page adds tangles and high feeding resistance. If the nozzle temperature reading itself swings during the print, see our guide to a fluctuating nozzle temperature.

Prusa's heat creep article (MK2.5, MK2.5S, MK3, MK3S and MK3S+) describes the same silent pattern for heat creep: "If your printer suddenly stops extruding filament, yet continues the print moves without reporting any error, you may be a victim of a heat creep." Prusa adds that heat creep is only one way a hotend can clog, and "most often than not, it is not the most probable explanation."

Step 1: Check the spool and the filament end

Simplify3D says to start with the obvious: "verify that you still have filament leading into the nozzle." If the spool has run out, load a new one.

If there's filament on the spool but the end has disappeared into the tube, it may have snapped. Bambu's "Filament break in the path" page lists three places brittle filament breaks: "Inside the AMS," "Inside or near the tool head," and "At the position where the bending of PTFE tube is large." The same page says "the filament can break inside the tool head, if it not dry enough."

On the X1, P1 and H2D, Bambu says the extruder's detection switch is a magnet and a Hall sensor. If filament breaks near the switch, it can stay triggered after unloading, and the printer reports that the slot's filament "may be broken in the toolhead." Bambu's fix starts with removing the switch to look for broken filament and checking the magnet springs back. Bambu says to "always power off the printer before performing any maintenance."

If filament keeps snapping, read our brittle filament guide and wet filament guide before you load the next spool.

How runout sensors and resume work

Both makers document what their printers do when a sensor sees the filament end. Follow the steps for your model.

Prusa: CORE One, CORE One L, XL, MK4/S, MK3.9/S, MK3.5/S and MINI

Prusa says the message "Filament runout during print, please insert new one" appears when "the printer detects that the filament has run out during a print. The sensor has triggered a pause, and the printer is waiting for new filament to be inserted." Prusa calls this "a normal and expected function."

To resume, Prusa says:

1. Insert new filament when prompted. Follow the on-screen instructions, and the printer will heat up and guide you through the change.
2. Make sure the new filament is fully loaded and extruding correctly before you confirm.
3. Press Continue on the printer's LCD to resume.

If you get runouts when there's still filament, Prusa says to "check the filament sensor wiring, cleanliness, and whether the filament path is smooth and free of friction."

A runout can only be caught if the sensor is on. When it's off, these printers show "Filament sensors are disabled. Enable them manually after printing." Prusa says to turn it back on in LCD Menu, Settings, Filament Sensor.

Bambu Lab: external spool

For the X1 series, Bambu's HMS code 07FF-2000-0002-0001 reads "External filament has run out; please load a new filament." Bambu says it triggers "when printing with spool holder filament" and the filament runs out. Its fix: "Feed the new filament into the extruder and click the 'retry' (or 'resume', depending on the firmware version) button on the screen."

Bambu's H2 series page for the same fault (07FE-2000-0002-0001 for the left extruder, 07FF for the right) describes what you'll see: the print pauses, "the toolhead is moved to the purge chute," a pop-up says the external spool is used up, and the extruder icon on the filament page "turns gray, it is normally green." If you find filament at the toolhead but the icon stays gray, Bambu says the sensor may be giving a false alarm and to check the Hall sensor and its magnet.

Bambu Lab: AMS and AMS HT slots

On the X1 series page for HMS 0700-2000-0002-0001 ("AMS1 Slot1 filament has run out"), Bambu says the warning appears when "the filament detection switch of the extruder detects that there is no filament left." The fix is to "insert the required filament and press the 'Retry' button on the screen/APP/Slicer to continue." If the message keeps coming back, Bambu says to contact its after-sales team. The same page lists equivalent codes for AMS units A to D and AMS HT units A to H.

Bambu Lab: AMS lite (A1, A1 mini)

Bambu's A1 series HMS page lists the runout codes 1200-2000-0002-0001 ("AMS1 Slot1 filament has run out") to 1203-2300-0002-0001. Bambu's AMS lite FAQ says the AMS lite "will automatically switch to another spool with the same filament properties when the current filament runs out."

Step 2: Look for a tangle or a dragging spool

If the printer kept moving with nothing coming out, check whether the spool turns. Prusa says a knot forms when the filament end is let go, "causing the filament end to go under another loop." Our tangled filament guide covers how to free one.

A spool doesn't need a knot to stop feeding. For the A1 series, Bambu lists what it checks after a tangle alarm with an external spool:

• the filament on the spool is wound and tangled
• the spool is placed directly on the table or in a drying oven
• the PTFE tube is too long or badly bent

Some Bambu printers watch for this:

• A1 and A1 mini. Bambu says the tangle detection module is enabled by default in Settings, Print Options. Without the AMS lite, a trigger makes the printer unload and pause. With the AMS lite, it first extrudes a little at the purge wiper to see if the module springs back. Bambu says it doesn't support "TPU, TPE, wet water-soluble filaments, and PVA filaments."
• P2S. Bambu says the P2S has air printing detection and filament tangle detection, enabled by default. With an AMS, if the G-code keeps asking for filament while the AMS odometer doesn't move, the printer cuts the filament, retracts it, pauses and reports an error. Bambu notes the extruder motor check "cannot distinguish" a clog from an external spool tangle, because both cause high resistance.

Bambu's guide to tangled or stuck filament offers a way to keep the print: cut the filament at the inlet, tap "Finished, Continue," wait for the cut end to run out, reinsert the filament and tap "Resume." Bambu says to hold the tangled filament while cutting "to prevent it from springing back as the tension releases." AMS owners should also read our AMS feeding problems guide.

Step 3: Check whether the gear ground the filament

Simplify3D says that if the drive gear keeps spinning while the filament can't move, it can grind away "enough plastic from the filament so that there is nothing left for the gear teeth to grab on to." It says you will "typically see lots of small plastic shavings," and may notice "the extruder motor is spinning, but the filament is not being pulled into the extruder body."

Simplify3D's suggestions, in its order, are to reduce retraction speed (for example by 50% as a test), raise the nozzle temperature by 5 to 10 degrees, slow the print down, and then check for a partial clog. Simplify3D describes grinding as what happens when "the filament is unable to move, yet the drive gear keeps spinning," so check the spool and the nozzle too. Our extruder clicking guide covers the sound and each maker's checks.

Step 4: Is it a clog or heat creep?

If filament reaches the extruder and the spool turns freely, the blockage is probably in the hotend. Polymaker lists "Filament not coming out at all, even though the extruder motor is turning" among the signs of a clog. It says to stop the print, cool down safely, and "avoid forcing the extruder; this can strip filament or damage components."

Simplify3D says that for a clog that starts partway through a print, check "that the filament is clean and that there is no dust on the spool," because enough dust "can cause a clog as it builds up inside the nozzle."

For heat creep, Bambu's heat creep page describes a real failure, likely from a hot enclosure or a faulty hotend cooling fan, where "the printer started printing normally. However, after some time, the nozzle developed a partial clog." For the X1 Carbon and P1S, Bambu calls "heat buildup inside the enclosure and heat creep inside the extruder" the most common reason for hotend or extruder clogs.

• To clear the clog, follow your maker's procedure from our clog diagnosis guide.
• To stop it happening again, see our heat creep guide for fans, chamber temperature and settings.

Overheated extruder motor driver

Simplify3D lists one electronic cause. The extruder motor works hard, and if the electronics aren't cooled well, the motor driver can overheat and hit its thermal cutoff. In that case, Simplify3D says, "the X and Y axis motors will be spinning and moving the extruder toolhead, but the extruder motor will not be moving at all." Its fix is to turn the printer off and let the electronics cool. It adds that an extra cooling fan may help if the problem continues. Before changing anything inside the printer, check your maker's documentation for your model.

What should you check before you resume?

Checklist

• Note whether the printer paused with a message or kept moving.
• Check there's filament on the spool and leading into the extruder.
• Look for broken pieces of filament in the tube, AMS or extruder.
• Turn the spool by hand and look for a knot or a loop caught under another.
• Check the spool isn't sitting on the table or in a box, and the tube isn't long or sharply bent.
• Look for plastic shavings around the drive gear.
• Heat the nozzle and try to push filament through by hand.
• Think about room and chamber temperature if it stopped partway through.
• On a Prusa, confirm the filament sensor is enabled.
• Inspect the print before you resume.

How this guide was checked

This is a research-based guide. We haven't reproduced these failures for it. Every cause, setting and menu path comes from the Simplify3D, Prusa Research, Bambu Lab and Polymaker pages listed in the sources, retrieved on 2026-10-11. Runout and resume steps are kept to the models each maker's page names. Read more in our editorial standards.

Print problems: what to check before you buy ↗