First layer too high, too squished or uneven on MK4, MK4S or CORE One (load cell)

Symptom

On an MK4, MK4S or CORE One, which calibrate the first layer by themselves with the load cell in the print head, the first layer comes out too high (round lines with gaps that do not stick), too squished (ridges, dragged plastic) or different from one area of the bed to another. The CORE One may also show an uneven bed warning.

Likely causes, from most to least likely

1. The nozzle is not seated correctly: after replacing or handling it, it ended up lower than it should (not pushed all the way up, loose thumb screws or a badly fitted nozzle adapter).
2. The automatic starting point does not quite suit your sheet or filament and a fine height adjustment is needed (Live Adjust Z or the Z offset in the PrusaSlicer profile).
3. The load cell is not measuring properly: overtightened fan or heatsink bolts that distort the reading, a damaged connection or a damaged white pad on the load cell.
4. The bed expansion joints are oriented the wrong way, which makes the first layer uneven.
5. On the CORE One, the bed is misaligned: something under the bed, the bed not resting on its rear stop, the bed cable in the way or trapezoidal nuts not running freely.

Checks to tell which one it is

• Run the load cell test (Control → Calibrations & tests → Loadcell test) and tap the nozzle only when the screen asks: the progress bar must fill evenly.
• With the printer off and cold, check that the nozzle is pushed all the way up and that the thumb screws on the heatbreak side are tight.
• Look at the first layer: if it fails the same way across the whole bed it is a height issue; if it changes from one area to another, suspect bed alignment or the expansion joints.
• On the CORE One, check whether the uneven bed warning (Uneven bed, #31111) appears.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Fine-tune the height

   While the first layer prints, long-press the screen knob to open Live Adjust Z and correct little by little (more negative = more squished). This adjustment is not kept for the next print; to make it permanent, set the value in PrusaSlicer under Printers → General → Z offset (a negative value squishes the first layer more).

   Check: The first layer lines are slightly flattened, joined together and without ridges.

   If that does not fix it: If you need a large correction, or a different one every time, stop compensating: check the nozzle and the load cell (next steps).

2. Seat the nozzle properly

   Turn the printer off and let it cool down completely. Loosen the thumb screws on the heatbreak side, push the nozzle up until it stops and tighten the screws again by hand only, without tools. If you use a nozzle adapter, check that it is fitted as Prusa describes and has not lowered the nozzle.

   Check: The nozzle does not move and the first layer is no longer squished or dragged.

   If that does not fix it: Run the load cell test (next step).

3. Load cell test

   Go to Control → Calibrations & tests → Loadcell test and follow the screen: tap the nozzle only when asked and do not touch anything else on the printer.

   Check: The progress bar at the top of the screen fills evenly.

   If that does not fix it: If the bar does not fill or fills in jumps, follow Prusa's load cell guide for your model: with the printer off and cold, slightly loosen the hotend fan bolts and, if that is not enough, the three heatsink bolts that hold the load cell, because overtightening distorts the reading.

4. Check the load cell and its connection

   If the test still fails, with the printer off, unplugged and cold, check the load cell connector and cable and, when you remove the print-head case, look (without touching it) at the white pad on top of the load cell for damage.

   Check: After reassembly the load cell test passes and the bar fills evenly.

   If that does not fix it: The load cell is built into the heatsink: if it is damaged, Prusa says to replace the heatsink assembly. Contact Prusa support.

5. Bed expansion joints

   Check the orientation of the bed expansion joints: according to Prusa, all of them must have the full side facing the centre of the carriage.

   Check: After correcting them, the first layer is the same over the whole surface.

   If that does not fix it: If the unevenness remains and you have a CORE One, continue with the bed alignment.

6. CORE One: align the bed

   If the uneven bed warning appears, run Control → Calibrations & tests → Z alignment calibration. Then, with the printer off, look for objects under the bed (a spare nozzle, for example) and check that the bed rests on its rear stop, that the bed cable runs behind the rear Z axis and that the trapezoidal nuts move freely along the rods.

   Check: The uneven bed warning does not come back and the first layer is even.

   If that does not fix it: If a trapezoidal nut is worn, replace it; if the problem continues, contact Prusa support.

Prevention

• After changing the nozzle, always check that it is pushed all the way up with the screws hand-tightened.
• Do not handle the print head during the load cell test unless the screen asks you to.
• Use the Z offset in the PrusaSlicer profile instead of correcting by hand on every print.

Still not fixed?

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Printers it applies to

Prusa MK4, MK4S and CORE One.

Official sources

This guide summarises the official documentation linked here:

• First layer issues · Prusa Research
• Live adjust Z · Prusa Research
• Loadcell troubleshooting · Prusa Research
• Uneven bed #31111 (CORE One) #35111 (CORE One L) #36111 (CORE One INDX) · Prusa Research