Nozzle does not have round section

What happened?

This warning appears when the printer detects that the nozzle may not have a proper circular (round) cross-section. The printer shows the following message: "Nozzle doesn't seem to have round cross section. Make sure it is clean and perpendicular to the bed".

This check is part of the printer’s automatic first layer calibration routine. It can fail if the nozzle is dirty, damaged, clogged, or installed at a slight angle. A non-circular nozzle shape affects filament extrusion and first-layer adhesion.

Error name: Nozzle does not have round section

Error code: #17823 (XL) #28823 (MK3.5S) #23823 (MK3.5) 

How to fix it?

WARNING: operate with care, heated parts can cause severe burns. 

Inspect the nozzle for any filament residue or hardened material on the tip. Clean thoroughly using a brass brush. 

1. If you have a silicone sock on the hotend, remove it.
2. On the printer screen, go to Control -> Temperature and set the nozzle temperature to 250°C.
3. Wait at least 5 minutes. The remains of the filament must be warmed up slightly so that they can be removed more easily.
4. Restart the printer by pressing the Reset button. This ensures the printer is not actively heating, which reduces the risk of accidental short circuits during cleaning. 
   Avoid contact of the brush with the hotend cables and cartridges, as this could cause a short circuit, even if the printer is not actively heating. 
5. Using the brass brush, carefully clean the heaterblock and the hotend from the filament residue. 
6. When the heaterblock and the hotend are perfectly clean, wait until the hot parts are cooled down to ambient temperature. It takes approximately 10 minutes. 

Original Prusa XL	Original Prusa MK3.5/S

Check the nozzle position (XL)

On single-tool XL, after removing the x-carriage cover, hold the heater block (make sure that the nozzle is not heated!), and use the Torx TX8 key as shown in the picture to tighten the set screw. Do not use extra force while tightening, it may damage the hotend tube. The nozzle must be approximately at a 35°-40° angle to avoid damaging the hotend cables.

On multi-tool XL, identify the tool that has the nozzle to be checked, and park it. Hold the heater block (make sure that the nozzle is not heated!), and use the Torx TX8 key in the part shown in the picture to tighten the set screw. Do not use extra force while tightening, it may damage the hotend tube. The nozzle must be approximately at a 35°-40° angle to avoid damaging the hotend cables.

Check the nozzle position (MK3.5/S)

Look at the extruder from below and check the hotend position:

• Correct position: only the tip of the nozzle should be visible.
• Incorrect position: the heatsink is too low. The heaterblock is below the fan shroud.
• Repeat the assembly and make sure that the heatsink is seated in the top notch of the plastic part and both notches of the plastic part are filled.