6 sourced reports, with what to check before you buy. Each one links to the page it was reported on.
Costly or dangerous
Firmware updated via Fluidd bricks the Neptune 4 series printer
Elegoo's own wiki documents that updating firmware via Fluidd (rather than the official fix-pack procedure) is incompatible with the customized Klipper build and bricks the printer. Recovery requires an eMMC adapter, USB drive, and TF card — significant technical effort for a non-technical owner. This came from the 'Elegoo Neptune 4 Max error code' search.
What to check. Never update Neptune 4 series firmware via Fluidd. Use only Elegoo's official fix-pack + motherboard/screen firmware procedure, in the correct order (fix pack first, then motherboard, then screen).
MCU shutdown 'ADC out of range' from damaged thermal sensor
Elegoo's official wiki states the 'MCU mcu shutdown: ADC out of range' error on the Neptune 4 Plus/Max is most likely caused by a damaged thermal sensor, with bent cable pins as a secondary cause. This is a safety shutdown that halts printing. This came from the 'Elegoo Neptune 4 Max error code' search.
What to check. If this error appears, first re-seat the sensor cable and check for bent pins (2.0mm Allen key needed). If the issue persists, the thermal sensor likely needs replacement. A damaged sensor can cause unsafe temperature readings, so do not ignore it.
Nozzle rammed into bed during calibration due to faulty firmware, bending heatsink
A verified buyer on 3DJake (German) reports that upon arrival, the Neptune 4 Max rammed its nozzle into the print bed during calibration due to faulty Elegoo firmware, bending the heatsink and making the printer unusable out of the box. The owner had to buy a replacement hotend. This came from the 'Elegoo Neptune 4 Max problems thread' search.
What to check. When first powering on a new Neptune 4 Max, watch the first calibration/homing cycle closely. If the nozzle appears to plunge toward the bed, power off immediately. Consider checking the firmware version before first use.
'Endstop Z still triggered after retract' error halts homing after ~5 prints
The same verified 3DJake buyer reports that after only four successful prints, the Neptune 4 Max began failing with 'Endstop Z still triggered after retract', causing homing to abort. The owner spent countless hours emailing Elegoo support without resolution. This came from the 'Elegoo Neptune 4 Max problems thread' search.
What to check. If this error appears, check the Z endstop wiring and sensor alignment. Be aware that Elegoo support may not resolve this quickly based on this owner's experience.
MCU shutdown 'Rescheduled timer in the past' from third-party plugins or config conflicts
Elegoo's official wiki identifies two causes for the 'MCU mcu shutdown Rescheduled timer in the past' error on the Neptune 4 series: third-party plugin installation overloading the MCU, or incorrect firmware settings causing configuration conflicts. The fix requires loading a fix pack and updating both motherboard and touch screen firmware. This came from the 'Elegoo Neptune 4 Max error code' search.
What to check. Avoid installing third-party Klipper plugins on the Neptune 4 Max, and be cautious when modifying firmware settings. If the error appears, follow Elegoo's official fix-pack procedure.
Printer crashed due to insufficient hotbed heating in 19°C room
An Alza.cz verified buyer reports the printer crashed once because the platen could not reach its target temperature in a 19°C room — it struggled to get above 76°C. This appears to be an environmental limitation rather than a hardware defect, but it caused a print failure. This came from the 'Elegoo Neptune 4 Max common problems' search.
What to check. In cooler environments, allow extra warm-up time for the hotbed and consider enclosing the printer or raising room temperature. The hotbed may struggle to reach high temperatures in cold rooms.
These reports come from the pages linked under each item: owner forums, repair guides and workshops. They are not a diagnosis of any one unit, and a careful check of the one you are buying still applies.
Text from the Kriko 3D printers catalog, version 0.1.1, under the CC-BY-SA-4.0 license.
