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, 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.

Fixing failed thermistor

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).

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.

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) may have a break, and will need to be replaced.