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Kobra S1 or Kobra 3: a print head fan does not spin or is noisy (error 11950 on the S1)

Guide based on the manufacturer's official documentation; usi3d has not yet verified it on a machine.

Technology: FDM · Brand: Anycubic · Symptoms: A fan does not spin or is noisy, The printer shows an error code or message · Updated on 10 October 2026

Symptom

On the Kobra S1 or Kobra 3 a print head fan, the part cooling fan (which cools the model) or the hotend fan, does not spin, runs unevenly or is noisy. On the Kobra S1, if the part cooling fan does not start, printing stops with «CODE: 11950 Printhead Model Cooling Fan Startup Abnormality».

Likely causes, from most to least likely

  1. Filament debris, dust or something else is blocking the blades.
  2. The fan cable or its extension cable is loose, badly connected, pinched or broken.
  3. The fan screws or cover are loose, or a cable rubs against the blades.
  4. Broken blades or a faulty fan.
  5. On the Kobra S1, poor contact or bent pins at the print head adapter board connector or, rarely, a fault on that board.

Checks to tell which one it is

  • On the Kobra S1, set the part cooling fan to 100 % from the touch screen: it must spin without odd noises.
  • With the printer off, look for filament strings or dust between the blades and check whether any blade is broken.

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. Kobra S1: check the part cooling fan extension cable

    Switch off and unplug. Press both sides of the air duct and pull the print head front cover outwards. Check that the part cooling fan extension cable is undamaged and fully connected.

    Check: When switched on and set to 100 %, the fan spins.

    If that does not fix it: Clean and inspect the fan (next step).

  2. Clean and inspect the fan

    With the printer off, remove filament debris, dust or whatever is blocking the blades, tighten the fan screws and cover if they are loose and make sure no cable rubs against the blades. If a blade is broken, replace the fan.

    Check: The fan spins quietly.

    If that does not fix it: On the Kobra S1, check the adapter board connector (next step); on the Kobra 3, replace the fan (steps 4 and 5).

  3. Kobra S1: print head adapter board connector

    Switch off and unplug. Remove the front cover, unplug the part cooling fan from the extension cable and, with the H2.0 key, remove the four screws of the print head rear housing. Take out the extension cable, check the adapter board pins for bending or deformation and reconnect it.

    Check: 11950 does not appear again.

    If that does not fix it: If it persists, the fan, the extension cable or the board may be faulty: replace the fan (next step).

  4. Replace the part cooling fan

    With the printer off and unplugged. Kobra S1: remove the print head housing, unplug the fan from the extension cable and remove the three screws holding the fan and the air deflector with a Phillips screwdriver; fit the new one and route the cable. Kobra 3: the print head housing comes off without screws; unplug the red fan cable and the black light cable, remove the two fan screws and fit the new one.

    Check: The new fan spins normally.

    If that does not fix it: If the error persists, go to the last step.

  5. Replace the hotend fan

    With the printer off and unplugged. Kobra S1: remove the front cover (unplug its fan) and the print head rear housing (2.0 mm key), unplug the hotend fan from the print head board, free its cable from the clips and remove its screws. Kobra 3 (duct fan): remove the housing, unplug the fan, thermistor and heater cables, remove the two hotend screws (2.5 mm), turn it over and remove the two fan screws.

    Check: Once everything is reassembled, the new fan spins normally.

    If that does not fix it: Go to the last step.

  6. Information for Anycubic support

    If the fault persists, send the shop or Anycubic support a screenshot of the error, the model and serial number, the firmware version, a video from the operation until the error appears, the steps you took and any parts you replaced.

    Check: Support tells you which part to replace.

    If that does not fix it: Follow their instructions.

Prevention

  • Clean the print head fan blades regularly.
  • After taking the print head or hotend apart, make sure the part cooling fan cable is fully plugged in and that the housing does not pinch it or let it rub the blades.
  • Do not touch the blades while the fan is spinning.

Still not fixed?

Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.

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Printers it applies to

Anycubic Kobra S1 and Kobra 3.

Official sources

This guide summarises the official documentation linked here:

This guide is general information: usi3d has summarised it from the official documentation linked here, and it may not match your model, its version or its firmware. If in doubt, follow the manual and the support of your printer's manufacturer. You decide what you do with your machine, and working on it may affect its warranty.

Products that may help

Chosen for the symptoms in this guide and the printers it applies to, not for the commission. Check that they work with your printer before buying.

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