What Does This Error Mean?

A Prusa nozzle clog is usually caused by carbonized filament residue from printing at too high a temperature or leaving filament in the hotend idle at temperature. Perform a cold pull first, then an atomic pull if needed to clear the clog.

Affected Models

Prusa MK4
Prusa MK3.9
Prusa MK3S+
Prusa MINI+
Prusa XL

Common Causes

Carbonized filament buildup inside the nozzle from high-temperature printing or idle heat
Switching filament types without a purge — incompatible materials blend and clog
Printing PETG or flexible filaments that are prone to oozing and heat-creep clogs
Dust or debris on the filament entering and accumulating in the nozzle
Partially retracted filament left in the hotend during a long pause solidifying inside the nozzle

How to Fix It

Perform a cold pull.

Heat the hotend to printing temperature, push filament through manually to seat it, then cool to 90°C for PLA (or 130°C for PETG). When the target temperature is reached, pull the filament out firmly and quickly in one smooth motion. The pulled filament tip should show the shape of the nozzle interior — repeat until it comes out clean.
Try an atomic pull with nylon or cleaning filament.

Load a short piece of nylon filament (Taulman 618 or similar) or a cleaning filament cartridge. Heat to 250°C, push through, then cool to 90°C and pull. Nylon performs a more aggressive cold pull than PLA and is excellent for removing carbonized residue.
Use an acupuncture needle to clear a full clog.

Heat the hotend to 30°C above the normal print temperature to soften any solidified material. Insert a 0.3 or 0.4 mm acupuncture needle or guitar string into the nozzle tip and gently push through. This breaks up hard clogs that cold pulls cannot dislodge.
Purge thoroughly when changing filament.

When switching between filament types, especially from PLA to PETG or back, purge at least 50–100 mm of the new filament at an intermediate temperature to flush the old material. In PrusaSlicer, use the Color Change or Filament Purge feature at layer changes for multi-material transitions.
Replace the nozzle if clogs recur.

If the same nozzle clogs repeatedly with fresh filament and normal temperatures, the nozzle bore may be scratched or deformed from a previous clog or a hard crash. A worn nozzle creates turbulence inside the bore that encourages material accumulation. Replace it — Prusa E3D-compatible nozzles cost under $5.

When to Call a Professional

If cold pulls and needle cleaning cannot restore flow, the nozzle may need replacement. Prusa nozzles are inexpensive consumables — the MK4 uses an E3D-style V6 nozzle and the MINI uses a 0.4 mm nozzle in the same format.