Custom Snapmaker U1 Firmware

Faulty Toolhead Bypass

This firmware adds two Firmware Config recovery options for Snapmaker U1 printers with a damaged toolhead:

• Bypass Thermistor: for error 0003-0523-0000-0002, when a toolhead’s MCU is alive and talking over USB but its thermistor is faulty. Based on Snapmaker’s official guide: https://wiki.snapmaker.com/en/snapmaker_u1/troubleshooting/bypass_a_faulty_toolhead
• Bypass MCU (Disconnected): for when a toolhead’s MCU itself is dead or its USB cable is disconnected entirely, so Klipper can’t talk to it at all. Stock Klipper treats this as fatal for the whole printer - without this bypass, one dead toolhead board takes down all four. It repoints that toolhead’s [mcu] at a software simulator (see Toolhead MCU Simulator) instead of the real board, so Klipper can still boot.

Warning

Both bypasses are only intended to let the printer boot and keep using the remaining toolheads until the damaged part is replaced or reconnected.

• Bypass Thermistor: do not heat or print with the faulty toolhead while the bypass is enabled.
• Bypass MCU (Disconnected): the toolhead’s stepper motor and heater are fully inert while this bypass is enabled - it cannot move, extrude, or heat at all, not just “don’t heat with it.”
• Disable the bypass again after the repair is complete.

Configure

1. Enable Advanced Mode on the printer screen.
2. Open http://<printer-ip>/firmware-config/.
3. Go to Settings -> Troubleshooting.
4. Use the row for each affected toolhead: Faulty Toolhead 1, Faulty Toolhead 2, Faulty Toolhead 3, or Faulty Toolhead 4.
5. Change that row from Normal to Bypass Thermistor (MCU alive, thermistor bad) or Bypass MCU (Disconnected) (MCU dead or cable unplugged).
6. Firmware Config will restart the toolhead simulator (if applicable) and Klipper automatically.

To undo it after replacing the faulty part, set each affected toolhead row back to Normal.

What Bypass Thermistor Changes

For the selected toolhead, the override:

• remaps the extruder temperature sensor_pin to PC5
• raises the extruder max_temp to 999
• sets the extruder max_power to 0.000001 because Klipper rejects 0
• sets the matching nozzle cooling fan heater_temp to 999
• sets the matching nozzle cooling fan fan_speed to 0
• changes the last stepped_temp_table entry from 260, 0.9 to 260, 0

Toolhead mapping:

• Toolhead 1 -> [extruder] and [heater_fan e0_nozzle_fan]
• Toolhead 2 -> [extruder1] and [heater_fan e1_nozzle_fan]
• Toolhead 3 -> [extruder2] and [heater_fan e2_nozzle_fan]
• Toolhead 4 -> [extruder3] and [heater_fan e3_nozzle_fan]

What Bypass MCU (Disconnected) Changes

For the selected toolhead, the override:

• repoints [mcu eN]’s serial at a dummy MCU simulator process instead of the (missing) physical USB serial device
• raises the extruder max_temp to 999
• sets the extruder max_power to 0.000001 because Klipper rejects 0
• sets the matching nozzle cooling fan heater_temp to 999
• sets the matching nozzle cooling fan fan_speed to 0
• changes the last stepped_temp_table entry from 260, 0.9 to 260, 0

Toolhead mapping:

• Toolhead 1 -> [mcu e0], [extruder], [heater_fan e0_nozzle_fan]
• Toolhead 2 -> [mcu e1], [extruder1], [heater_fan e1_nozzle_fan]
• Toolhead 3 -> [mcu e2], [extruder2], [heater_fan e2_nozzle_fan]
• Toolhead 4 -> [mcu e3], [extruder3], [heater_fan e3_nozzle_fan]

See Toolhead MCU Simulator for how the simulator itself works.

Implementation Note

Inference from the official guide and the stock U1 printer.cfg:

• The official guide says to change the temperature sensor pin suffix to PC5.
• Stock U1 configs use per-toolhead MCU pin names (e0:PA2, e1:PA2, e2:PA2, e3:PA2).
• This firmware therefore implements the remap as e0:PC5, e1:PC5, e2:PC5, or e3:PC5 for the selected toolhead.

The active override is installed into:

/oem/printer_data/config/extended/klipper/

Filename pattern:

• faulty_toolhead1.cfg / faulty_toolhead1_mcu.cfg
• faulty_toolhead2.cfg / faulty_toolhead2_mcu.cfg
• faulty_toolhead3.cfg / faulty_toolhead3_mcu.cfg
• faulty_toolhead4.cfg / faulty_toolhead4_mcu.cfg

Only one override is active per toolhead at a time - switching between Bypass Thermistor and Bypass MCU (Disconnected) removes the other override’s marker file. Different toolheads can have different bypasses enabled at the same time.