Neptune 3 Pro: levelling does not work, the first layer is uneven or the nozzle keeps going down
Guide based on the manufacturer's official documentation; usi3d has not yet verified it on a machine.
Technology: FDM · Brand: Elegoo · Symptoms: Unlevel or warped bed, Uneven first layer (thin and thick areas), Homing or a sensor fails (endstop, levelling) · Updated on 9 October 2026
Symptom
On the Neptune 3 Pro (and the rest of the Neptune 3 series) the first layer is uneven or does not stick even after levelling, or when homing Z the nozzle keeps going down without stopping because the proximity sensor does not trigger.
Likely causes, from most to least likely
- The PEI plate is missing or is not metal: the Neptune 3 series proximity sensor only detects metal.
- The proximity sensor is loose or tilted: it must be parallel to the plate to trigger.
- The nozzle sticks out too far because it was fitted incorrectly: the sensor detects at 4 mm and the nozzle gets in the way when homing.
- Play: loose X or Y eccentric nuts (the extruder or bed wobbles) or loose bed screws.
- The two screws holding the Z-axis brass nut are fully tightened and the axis binds when rising.
- The bed and the X axis are not parallel.
Checks to tell which one it is
- Check that the original PEI plate (or a metal one) is in place before homing, levelling or printing.
- Move the extruder and bed by hand: they must not wobble.
- With the printer switched off and unplugged, check that the proximity sensor screws are tight and the sensor sits parallel to the plate.
- Measure with a ruler the distance from each end of the X axis to the bed: it should be the same.
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
Fit the metal PEI plate
Do not press Home, run levelling or start a print without the PEI plate in place. If it is not the original, make sure it is metal.
Check: When homing, the nozzle stops above the plate.
If that does not fix it: Check for play (next step).
Remove play
If the extruder or bed moves, tighten the X and Y eccentric nuts with a spanner. Remove the PEI plate and tighten the four bed screws if they are loose.
Check: Neither the extruder nor the bed wobbles when moved by hand.
If that does not fix it: Check the sensor and nozzle (next step).
Check the sensor and nozzle
Switch off and unplug the printer. Disconnect the cable, remove the extruder screws and take it off; tighten the proximity sensor screws and set it parallel to the plate. Also check that the nozzle is fitted correctly and does not stick out too far.
Check: With everything refitted, Z homing stops at the right height.
If that does not fix it: Check the Z axis (next step).
Loosen the Z-axis brass nut
The two screws holding the Z-axis brass nut should stay slightly loose. If they are fully tightened, loosen them so the axis does not bind when rising.
Check: The Z axis moves up and down smoothly without binding.
If that does not fix it: Check that the X axis is parallel (next step).
Make the X axis parallel to the bed
With the printer off and unplugged, if the X axis is not parallel to the bed, adjust it following Elegoo's video. Then load the latest firmware Elegoo indicates, set the Z offset again and re-level.
Check: The first layer comes out even across the whole plate.
If that does not fix it: If the nozzle still keeps going down, switch off as soon as you see it so it does not crash: the sensor is probably damaged and needs replacing.
Prevention
- Always keep the metal PEI plate in place.
- Check the eccentric nuts and bed screws every now and then.
- After any mechanical adjustment, set the Z offset again and re-level.
Still not fixed?
Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.
Printers it applies to
Elegoo Neptune 3 Pro, Neptune 3 Plus and Neptune 3 Max.
Official sources
This guide summarises the official documentation linked here:
This guide is general information: usi3d has summarised it from the official documentation linked here, and it may not match your model, its version or its firmware. If in doubt, follow the manual and the support of your printer's manufacturer. You decide what you do with your machine, and working on it may affect its warranty.