Neptune 4 TL4970 and Neptune 4 Pro TL4972 - MCU shutdown: ADC out of range

Issue 

Printer displays a Klipper error screen after starting that says MCU mcu shutdown: ADC out of range.

Cause

The thermistor (temperature sensor) in the hot end is reporting an abnormal temperature - it should be room temperature for a printer that has just been started. The thermistor is either not connected, has broken wires, or has reached end-of-life. This is a consumable part with a 3-month warranty, like the nozzle or heater cartridge. 

If you have another working Neptune 4 or 4 Pro available to confirm diagnosis: power off & remove the faulty printer's tool head from the gantry as described here, temporarily connect it to the other printer's extruder data cable, and power it on. 

If the other printer now has the same ADC error, the hot-end thermistor needs to be replaced. If the other printer does not get the ADC error and starts normally, the faulty printer's extruder data cable has a break or it's motherboard plugs are not connected properly - see the bottom of the page for a fix.

Which printer version do I have?

The Neptune 4 TL4970 and Neptune 4 Pro TL4972 printers have two hardware versions identifiable by different print head cable types. They also have different thermistors.

Ribbon-cable version - Serial number will be like NEP 4 .xxxxx.xxxx / NEP 4 Pro .xxxxx.xxx:

The thermistor for this version is Cat No. 001433 

Type-C version - Serial number will be like NEP 4+.xxxxx.xxx / NEP 4 PRO+.xxxxx.xxx:

The thermistor for this version is Cat No. 001434

Replacing the thermistor - Ribbon cable version

Remove the printer tool head from the gantry - see the entire process here.

Disconnect the printer tool head black-sleeved flat data cable.

Disconnect and remove the cooling fans from the tool head.

Check the white thermistor cable is plugged-in securely and has no breaks in it.

If it is OK, unscrew the retaining screw for the thermistor, and unplug the white thermistor cable.

Replace the thermistor (Cat No. 001433 - ribbon, 001434 - Type-C ).

You can also replace the hot end (Cat No. 001643) instead.

Plug in the white thermistor cable.

Connect the tool head data cable.

Turn on the printer to confirm that the error message has cleared.

Turn off, reinstall and reconnect the fans.

Reinstall the printer tool head to the gantry.

Level the printer and set the Z-Axis offset using a piece of paper.

Replacing the thermistor - Type-C version

Remove the printer tool head from the gantry - see the entire process here

Undo the two 1.5mm screws & disconnect the printer tool head USB-C data cable

Disconnect and remove the cooling fans from the tool head

Undo the 2x 2mm screws & remove the hot end from the tool head

Slide off the silicone sock

Slide off the metal spring sleeve

Take out the thermistor

Install a new thermistor into the hole

Slide the sleeve back on - the thermistor wire goes in the channel in the metal sleeve

Slide the silicone sock back on 

Screw the hotend back into the tool head (don't forget the blue PTFE tube)

Connect the heater and thermistor cables

Connect the tool head USB-C data cable.

Turn on the printer to confirm that the error message has cleared.

Turn off, reinstall and reconnect the fans.

Reinstall the printer tool head to the gantry

Level the printer and set the Z-Axis offset using a piece of paper.

Error message is still there after replacing thermistor

Remove the bottom cover plate over the motherboard and check all plugs are seated correctly. If they are, the extruder data cable (Cat No. 001465 - ribbon version) may have a break, and will need to be replaced.