Kobra S1: little or no filament comes out (hotend clog or error 11518)

Symptom

On the Kobra S1 the extruder motor turns but little or no filament comes out, the extruder clicks, or the extruded line is thinner than normal and the walls have gaps. Sometimes the print pauses with «CODE: 11518» (filament clog) and the screen asks you to retract the filament above the print head before tapping «Resume Print».

Likely causes, from most to least likely

1. Material residue built up inside the hotend after many hours of printing, which ends up blocking the nozzle.
2. Filament jammed inside the heat sink or the extruder, or the extruder gear grinding the filament.
3. An ACE Pro buffer mechanism that does not spring back because filament is stuck inside it.
4. Nozzle temperature too low: the filament does not melt completely.
5. The filament: wet, deformed, tangled, out of diameter tolerance or so thin the gear cannot drive it; or filled with particles, such as carbon fibre or glow-in-the-dark filament.
6. The nozzle is damaged and no longer extrudes normally.

Checks to tell which one it is

• Pull each of the four buffers by hand: they must spring back to their position on their own.
• Do the feed test in step 4: if the filament comes out of the bottom of the heat sink, the extruder works and the clog is in the hotend; if it does not get through, it is in the heat sink or the extruder.
• Look at the filament: bent or swollen tip, brittle or wet strand, tangles or resistance before it enters the print head.

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. Retract the filament and resume (11518)

   If 11518 appears, do what the screen asks: retract the filament above the print head and tap «Resume Print». Check that it retracts without resistance and does not stay caught at the print head inlet.

   Check: Printing continues and the error does not come back.

   If that does not fix it: Check the filament and the buffer (next steps).

2. Check the filament

   Cut off the section with the deformed tip and reload it. If it is tangled or the spool drags, free the filament path; if it may be wet, swap it for dry filament.

   Check: Loading and resuming work without errors.

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

3. Check the ACE Pro buffer

   With the printer off and unplugged, pull each buffer and see whether it springs back; if it does not, remove the filament stuck inside following Anycubic's buffer clog guide. If it is fitted incorrectly, remove the two screws of its cover with the H2.5 Allen key, line up all four buffers the same way and refit them.

   Check: All four buffers move smoothly and the filament passes without catching.

   If that does not fix it: Do the extruder feed test (next step).

4. Extruder feed test

   With the printer cold, remove the print head front cover, unplug the fan connector and take the hotend out by pushing the handle up. Set the nozzle to 250 °C, push filament through the PTFE tube at the back until you feel resistance and, in «Move Axis», tap E+ (it only works once the nozzle is at 250 °C) while gently pushing the filament.

   Check: You feel the extruder pulling the filament and it comes out of the bottom of the heat sink: the extruder is fine and the clog is in the hotend (step 6).

   If that does not fix it: If it does not get past the heat sink, check the heat sink and the extruder (next step).

5. Check the heat sink and the extruder

   Switch off and unplug. With the H2.0 key, remove the screws of the cutter assembly, taking care not to cut yourself, and look for debris or jammed filament. Then remove the three screws of the strain gauge assembly and check the heat sink. If nothing is there, take the extruder apart as shown in Anycubic's official Kobra S1 clog video.

   Check: The filament path is clear and the filament passes through the heat sink.

   If that does not fix it: Unclog the heat break and the nozzle (next step).

6. Unclog the heat break and the nozzle

   First try raising the nozzle slightly above the filament's temperature (Anycubic gives 250 °C for PLA) and tapping «Extrude». If nothing comes out, wearing gloves and with the hotend out, set it to 150 °C, hold the heat break with pliers and slowly push a piece of filament, or a long 1.5 mm Allen key, to force the clog out through the nozzle. Then raise it to 230 °C and pass the cleaning needle through the nozzle.

   Check: When you reload, the filament comes out of the nozzle continuously and steadily.

   If that does not fix it: The nozzle may be damaged: replace it with a new quick-swap nozzle. The 0.2 mm nozzle is harder to clean and may need several attempts.

7. Information for Anycubic support

   If nothing changes, contact the shop or Anycubic support with the model, the firmware version, photos or videos of the problem, whether you use the ACE Pro, the steps you have already done and anything changed recently (filament, nozzle, firmware or slicing profile).

   Check: Support tells you which part to check or replace.

   If that does not fix it: Follow their instructions.

Prevention

• Check how the nozzle extrudes once a month.
• Use filament that meets the official specifications and dry it if it has been stored for a long time.
• Print at the right temperature: with the nozzle too cold the filament does not melt completely and clogs.
• When you refit the hotend, make sure the print head is fully seated and the hotend cable is pushed all the way in.

Still not fixed?

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Printers it applies to

Anycubic Kobra S1.

Official sources

This guide summarises the official documentation linked here:

• Kobra S1 Abnormal hotend clogging · Anycubic
• Kobra S1 - Guide for Cleaning Hotend Clogging · Anycubic
• Kobra S1 - CODE 11518 Filament clogging detected · Anycubic