Uneven first layer: squished in some areas and gappy in others

Symptom

The first layer isn't the same across the bed: in some areas the lines are squished, almost see-through or ridged, and in others they are round, separated or not sticking. The whole layer may also come out too high or too low.

Likely causes, from most to least likely

1. Z offset or first-layer calibration set wrong: the whole layer is too high or too low.
2. Bed out of level or warped more than the bed mesh can compensate.
3. The mesh isn't applied: it isn't enabled in the start G-code, an old profile is loaded, or an adaptive mesh from another print is reused.
4. Unreliable measurement: inaccurate probe, dirty nozzle tip or bed, or a mesh measured at a different temperature from printing.
5. Sheet or nozzle changed, or printer moved, without redoing the calibration.

Checks to tell which one it is

• Print a test layer covering the whole bed and look at the lines: round with gaps means nozzle too high; flat, see-through or ridged means nozzle too low.
• If the difference follows a pattern (one side high, the other low), the bed is tilted or warped; if it is the same everywhere, it is the Z offset.
• In Marlin, check that the slicer start G-code includes M420 S1 to enable levelling; in Klipper, that the mesh is calibrated or loaded before printing (BED_MESH_OUTPUT shows the active mesh).
• In Klipper, PROBE_ACCURACY probes ten times: if the range exceeds 0.025 mm, the probe isn't accurate enough for levelling.

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. Clean the nozzle and the bed

   Carefully, with the nozzle hot, remove plastic residue from the tip; with the bed cold, clean it with isopropyl alcohol.

   Check: The tip is clean and the bed free of residue.

   If that does not fix it: Level the bed mechanically.

2. Level the bed mechanically

   If the bed has levelling screws or knobs, first level it at the corners with the paper method or your firmware's levelling tool. The mesh only approximates the bed's shape: it is no substitute for good mechanical levelling.

   Check: The paper test gives the same drag at every corner.

   If that does not fix it: Adjust the Z offset.

3. Adjust the Z offset

   Adjust the first-layer height with your printer's tool: Live Adjust Z on Prusa, babystepping or M851 on Marlin, PROBE_CALIBRATE with the paper test and SAVE_CONFIG on Klipper. Change it in small steps and watch the lines.

   Check: Lines are flat and joined, with no gaps or ridges.

   If that does not fix it: Measure the mesh again.

4. Measure the mesh under printing conditions

   Heat the bed to printing temperature and wait a few minutes for it to settle before measuring; always measure at the same temperature. On Klipper use BED_MESH_CALIBRATE and save the profile; on Marlin, G29 followed by M500 to store it.

   Check: The test layer looks the same across the whole bed.

   If that does not fix it: Check that the mesh is actually used.

5. Make sure the mesh is used

   On Marlin, add M420 S1 to the slicer start G-code to enable the saved levelling. On Klipper, load the profile with BED_MESH_PROFILE LOAD or calibrate the mesh at the start; don't reuse an adaptive mesh from another print.

   Check: The first layer follows the bed's shape in every area.

   If that does not fix it: Check the probe and the bed.

6. Check the probe and the bed

   If the probe isn't repeatable (on Klipper, PROBE_ACCURACY with a range above 0.025 mm), check it following the manufacturer's guide. If the bed is warped beyond what the mesh can correct, ask the manufacturer about replacing it.

   Check: Repeated probe measurements agree.

   If that does not fix it: Contact your printer manufacturer's technical support.

Prevention

• Redo the first-layer calibration after changing the sheet or nozzle and after moving the printer.
• Always measure the mesh at printing temperature.
• Keep the nozzle tip and the bed clean.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• First Layer Calibration (i3) · Prusa Research
• Bed Mesh · Klipper
• Probe calibration · Klipper
• Auto Bed Leveling · Marlin