5 sourced reports, with what to check before you buy. Each one links to the page it was reported on.
Costly or dangerous
Neptune 4 firmware updated via Fluidd bricks the printer (official Elegoo wiki)
Elegoo's own wiki states that updating the Neptune 4 series firmware through Fluidd is incompatible and bricks the printer. Recovery is not trivial: it needs an eMMC adapter, a USB drive, a TF card and a fix pack flash. This is the highest-consequence firmware risk on the machine.
What to check. Never update Neptune 4 firmware through Fluidd; use only Elegoo's official fix-pack and firmware update procedure. A bricked unit requires eMMC-level recovery, not a simple reflash.
Neptune 4 hotend and extruder not robustly built; cleaning nozzle breaks heater/temperature sensor
An owner reports having to replace the hotend twice after only a few prints, calls the extruder/hotend not robustly built, and says the heater or temperature sensor breaks when cleaning the nozzle. The filament detector is also reported as wobbly and falling out.
What to check. Budget for hotend replacement as a recurring cost; handle the nozzle and its wiring carefully because cleaning can damage the heater or thermistor. The extruder/hotend is the weak point on this machine.
Filament snaps and jams inside the Neptune 4 extruder gear, hard to clear
A French repair-forum thread describes filament snapping inside the extruder's toothed-wheel mechanism, leaving a stuck piece that is hard to remove because the gear does not easily disassemble; the user had to pry the gear off with a flat screwdriver.
What to check. If filament jams inside the extruder, the toothed gear may need to be pried off to clear it; the assembly is not designed for easy cleaning. This is a moderate, owner-reported maintenance headache.
Neptune 4 Plus/Max MCU shutdown 'ADC out of range' traced to damaged thermal sensor
Elegoo's official wiki attributes the 'MCU mcu shutdown: ADC out of range' error on the Neptune 4 Plus/Max to a damaged thermal sensor or damaged thermal wiring between hotend and heated bed. This is a shutdown-level fault that halts the printer.
What to check. On the Plus/Max models, a thermal-sensor or wiring fault triggers a full MCU shutdown; check the hotend/bed thermistor wiring and cable pins first. This is specific to the Plus/Max variants, not the base Neptune 4.
Neptune 4 firmware crashes after a period of inactivity (owner review)
A verified owner review on Alza reports the printer would not switch on after two days of inactivity, blaming firmware, and warns the unit must stay plugged into electricity or the firmware may crash. This corroborates the firmware fragility theme from Elegoo's own wiki.
What to check. Keep the Neptune 4 powered/plugged in rather than fully unplugged for long idle periods, and be prepared for firmware boot failures after inactivity.
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.
