Intermittent first layer under-extrusion in specific area of the print bed  

Intermittent first layer under-extrusion in specific area of the print bed

Hi everyone,

I'm experiencing a strange first layer issue on my Prusa MK4S and I'm running out of ideas.

Printer Setup

• Prusa MK4S Original Prusa High Flow (HF) 0.4 mm nozzle
• Original Nextruder
• Prusament PETG
• Nozzle temperature: 243 °C
• First layer speed: 40 mm/s
• First layer width: 0.45 mm
• Volumetric speed limit: 20 mm³/s
• Textured PEI sheet

The Problem

I'm getting visible under-extrusion on the first layer. The strange part is that it doesn't happen everywhere and often appears in roughly the same area of the print.

The first layer lines are present but there are many small gaps where not enough material seems to be extruded.

I stopped the print directly after the first layer to inspect it more closely. (First Layer Height is 0,2). Picture shows the side facing the print bed (turned around for photo).

Observations

• The issue occurs with multiple filament brands, not just one spool.
• The issue is not present on every print but occurs quite frequently.
• The affected area is often located in a similar region of the bed.
• The rest of the first layer looks good.
• Adhesion is generally fine.
• Higher layers usually print without obvious problems.
• The printer is fully calibrated and passes self-tests.
• What I've Considered Partial nozzle clog Extruder gear slipping Filament feed resistance Load cell / bed leveling issue Local bed flatness problem
• However, since the issue appears only sometimes, I'm unsure whether this is a mechanical extrusion problem or a first layer compensation / load cell related issue.
• Same print is perfect on another MK4S

Troubleshooting Already Performed

To rule out common causes, I've already tried the following:

• Checked the extruder gears for filament debris and buildup → no visible filament dust or debris.
• Performed the load cell test → passes without issues.
• Tested multiple filament spools (including different materials) → issue remains.
• Tested different nozzle temperatures → no significant improvement.
• Thoroughly cleaned the print sheet with dish soap and warm water.
• Also cleaned the sheet with 99% isopropyl alcohol.
• Tried a different textured PEI sheet → same behavior.
• Removed the print sheet and checked the heated bed for debris or contamination underneath → nothing found.
• Verified that the filament spool rotates freely and is not snagging.
• Printer passes self-test and calibration routines.

Questions

• Does this look more like under-extrusion or an incorrect nozzle-to-bed distance?
• Has anyone seen similar behavior on a MK4/MK4S with the HF nozzle?
• Could this be related to load cell probing or bed mesh compensation?
• Would you recommend replacing the HF nozzle with a standard 0.4 mm nozzle as a diagnostic step?

Any ideas or troubleshooting suggestions would be greatly appreciated.

Thanks!