What Does This Error Mean?

Most Prusa MMU3 errors are caused by filament tip shape issues — a bad tip after retract confuses the selector and causes jams. Reload the current filament manually using the MMU control menu and inspect the filament tip for strings or blobs.

Affected Models

Prusa MMU3
Prusa MK3.9 with MMU3
Prusa MK4 with MMU3

Common Causes

Poor filament tip shape after unloading — strings or blobs jam in the MMU selector
Filament buffer incorrectly routed, adding too much resistance to the filament path
MMU3 selector blade worn or misaligned, failing to cut cleanly between filament changes
Filament spool tangled, adding sudden resistance that triggers an MMU load error
Hotend temperature too low during filament change, leaving solidified material in the nozzle

How to Fix It

Manually unload and inspect the filament tip.

Use the LCD menu to manually unload the current filament from the MMU. Examine the tip of the unloaded filament — it should come to a clean tapered point. A stringy, oozing, or bulbous tip will jam in the selector on the next load cycle and must be trimmed off cleanly before reloading.
Re-route the filament buffer.

The MMU3 buffer must be routed so filament can move freely without binding or sharp bends. Follow the Prusa MMU3 documentation for the correct buffer routing for your setup. Too much resistance in the buffer path causes the MMU to report a load error even though the filament path is physically clear.
Clean the selector blade area.

Open the MMU3 selector cover (refer to the Prusa MMU3 assembly guide) and inspect for filament debris around the selector blade. Use tweezers to remove any filament fragments caught in the blade or the selector carriage channel. A single small fragment can prevent the blade from moving to the correct position.
Check for a tangled or stuck spool.

A tangled filament spool suddenly resists as the MMU tries to retract, generating an overload error. Inspect each spool on the MMU buffer — the filament should unwind freely from the outside of the spool. Untangle any loops and ensure the filament is not caught under a crossed-over section on the spool.
Increase hotend temperature during filament changes.

In PrusaSlicer's filament settings, increase the Tool Change Temperature by 5–10°C for each filament. A higher temperature during the change ensures the filament tip remains molten long enough to form a clean taper before retraction. This is the single most effective way to reduce MMU tip shape errors.

When to Call a Professional

If the MMU3 selector motor clicks or grinds and does not move the selector blade despite clearing all filament, the selector motor or gearbox may need inspection. Contact Prusa support or the Prusa community for repair guides.