[MK4S] PLA nozzle clogs after a few layers – tried everything, still stuck  

[MK4S] PLA nozzle clogs after a few layers – tried everything, still stuck

Hi all,

I’m reaching out here in the hope that someone can help me solve a really frustrating issue with my Prusa MK4S. For the past few weeks, I’ve been battling persistent nozzle clogs while printing PLA — and I’ve truly tried everything I can think of to fix it.

Here’s what’s happening:

Whenever I start a print, the first few layers usually go just fine, especially when it’s the first print after turning the printer on. The lines look okay, extrusion seems normal — but then, a few layers in, the printer starts struggling. The extruder motor begins clicking, and filament stops extruding completely. At that point, the nozzle is clogged, and the only way to continue is to unload and try again.

I thought it was bad filament at first, but I’ve now tested multiple high-quality PLA brands, all properly stored and dry. The issue happens with every single one.

Thinking it might be a worn-out nozzle, I replaced it — not once, but three times:

The stock nozzle that came with the MK4S upgrade kit

Two E3D High Flow Brass 0.4mm nozzles

Still no change.

Then I went even further:

I ordered and installed a completely new Nextruder hotend assembly, including:

A new heater block

A new thermistor

A new heatbreak

A new nozzle

This entire section of the printer is now essentially brand new. But… the problem still persists.

I also performed a cold pull (multiple times), adjusted the extruder tension screws, verified the fan function and self-tests, recalibrated the Z-offset, and followed Prusa’s gearbox calibration guide to the letter. Nothing helped.

Loading filament even manually feels tough — there’s clearly more resistance than there should be. And what really stood out: the filament often curls at the nozzle tip and sticks to it, even when loading or extruding during prep.

The strange thing is: it always starts well. It just fails progressively as the print continues. That makes me wonder — could this be heat creep, even though the fans are working fine and I’m not printing inside an enclosure?

At this point, I’ve even completely disassembled the entire Nextruder, inspected everything, and reassembled it from scratch. Still the same issue.

To summarise what I’ve tried:

Replaced the nozzle (3x)

Replaced the full hotend

Replaced the thermistor

Tried various brands of PLA

Performed cold pulls

Disassembled and reassembled the extruder

Verified fan function and cooling

Gearbox calibrated and extruder tension adjusted

Adjusted Z-offset and print temps (tested 210–235°C)

Clean bed, open environment, no enclosure

I’m honestly out of ideas. I don’t want to keep replacing parts without understanding the root cause. The fact that even loading filament requires force, and that it only works briefly before failing, tells me something’s not right — but I have no clue what.

Has anyone seen this kind of behavior before? Any idea what else I can check or try?

Thanks so much for reading and for any advice you can give.

Jan-Martijn