Elegoo has not published a numbered error code list for the Neptune 4. The printer ships with Klipper firmware, and when a fault trips, the touchscreen and the Fluidd web interface show the same plain-language shutdown message Klipper generates internally, naming the stepper driver, thermistor or MCU that caused the stop. That full string, including the technical text after the colon, is the only reliable lookup key. Write it down exactly, note whether it happened during homing, heating, leveling or mid print, and check the firmware version shown under Settings, About Printer before searching further or contacting Elegoo.
The base Neptune 4 and the Neptune 4 Pro share the same Klipper mainboard generation and firmware track. Elegoo’s wiki reflects that: most of the fault pages for this pair are titled “Neptune 4/4 Pro” rather than split by model, which is a reasonable sign the underlying firmware behavior is the same on both. The Neptune 4 Plus and Neptune 4 Max sit in a separate FAQ section on Elegoo’s wiki, because those two larger printers use a different hotbed wiring layout and a different homing and leveling sequence. A fault that names a proximity switch failing during homing, a V-wheel-driven hotbed, or a Z-offset routine specific to the Plus/Max leveling process is not a confirmed match for the base Neptune 4, even if the wording looks close. Where a difference between the base Neptune 4 and the Pro actually changes the fix, it’s called out below.
Stepper driver overheat warnings
TMC stepper_x: OVER temp warning shows up when the X-axis stepper driver reports a temperature above its safe limit. Elegoo’s fix path starts with the fan under the base unit, which should start spinning once the nozzle passes roughly 50 C; if it doesn’t turn, open the bottom cover and clear whatever is blocking it. After that, check whether the X or Y belt is over-tightened, since excess tension makes the motor work harder and generates more heat, and reseat the motherboard fan connector with the printer powered off. A related message, TMC stepper_y_reports error: DRV STATUS: 401b0103 otpw=1(OvertempWarning!), points to the same driver-level overtemp condition on the Y axis and gets the same checks. If the warning returns after all of that, Elegoo asks for the printer’s serial number so it can confirm you’re on the matching firmware build.
MCU shutdown and thermal sensor faults
Mcu: mcu shutdown, ADC out of range fires when a hotend or hotbed thermistor reports a reading outside its configured minimum or maximum, which usually means the sensor is disconnected, the wire has broken internally, or the connector at the print head or mainboard has worked loose. Power the printer off, move the bed and toolhead gently to see if flexing the cable near the connector reproduces or clears the fault, and reseat the print head flat cable before reaching for a replacement thermistor. If the nozzle or bed temperature reads a negative number on screen, that’s consistent with a broken sensor circuit rather than a bad connection. Two related strings, Error message: MCU rpi shutdown: Unable to open spi device. and Error: unable to open config file, both point to a communication or configuration problem between the Klipper host and the mainboard rather than a thermal issue; Elegoo’s fix pack reinstall is the documented path for both, and neither should be worked around by hand-editing Klipper config files you don’t recognize.
Startup and firmware mismatch faults
If the touchscreen sticks on the loading screen and the motherboard firmware version shows as missing under About Printer, Elegoo’s own troubleshooting for “Neptune 4 & Neptune 4 Pro” starts by powering off and reconnecting one ribbon cable at a time to isolate which connection is preventing the screen from reaching the menu. The touchscreen firmware and the motherboard firmware are matched pairs; if either is out of date relative to the other, the printer can boot into an unstable state or throw an error immediately on power-up, which is also the description Elegoo gives for its separate “An error occurs once turned on” entry. The fix in both cases is reflashing the matching fix pack from Elegoo’s download center, using a FAT32 USB drive under 16 GB, never through Fluidd, since Neptune 4 series printers run a custom Klipper build that Fluidd’s update path isn’t built for and can brick the board. A blank, unresponsive touchscreen with no error text at all is treated as a separate symptom in the same family and gets the same cable-reseat-then-reflash sequence before a screen swap is considered.
Leveling, extrusion and filament warnings
The base Neptune 4 uses a 121-point (11×11) automatic bed leveling routine driven by an inductive proximity switch at the nozzle, the same sensor type used across the whole Neptune 4 lineup. Two FAQ entries worth checking here are leveling point parameters not being saved between sessions, and the nozzle sitting too far from the heated bed during auxiliary leveling, both of which Elegoo treats as leveling data or Z-offset issues rather than sensor failures, and both point back to re-running the leveling routine after confirming the Z-offset is set correctly. A filament runout switch that triggers with filament still loaded is listed as a common false-positive tied to the sensor’s mechanical position rather than a wiring fault, and Elegoo’s FAQ also separately documents the extruder’s load and unload buttons appearing to work in reverse, which is a UI mapping question rather than a mechanical one. None of these produce a Klipper shutdown message; they show up as behavior you notice rather than text on screen, so they won’t match a specific quoted string.
What differs from the Neptune 4 Pro
The base Neptune 4 uses POM plastic V-guide wheels on its linear rails, while the Pro uses all-metal U-shaped steel wheels on both X and Y. The base Neptune 4 has a single 250 W heated bed with one thermistor, while the Pro splits its 250 W bed into two independently controlled zones, roughly 150 W center and 100 W outer, each with its own sensor, which is why some Pro-specific thermal FAQ entries reference a zone that doesn’t exist on the base model. Neither printer ships with an onboard accelerometer for input shaping calibration out of the box; Elegoo’s wiki confirms the Neptune 4 Pro has an ADXL345 mounted on the toolhead, while the base Neptune 4 supports input shaping in firmware only if you wire your own ADXL345 to the motherboard’s documented interface pins. If a resonance-testing or accelerometer-related warning shows up on a base Neptune 4 that hasn’t had a sensor added, that’s expected, not a fault. Elegoo has not published a marketing claim of a ceramic bed heater on the base Neptune 4; that two-zone ceramic layout is specific to the Pro’s bed assembly.
The larger Neptune 4 Max runs the same Klipper firmware track; see the Elegoo Neptune 4 Max Error Code Index for its documented fault set.