After a startup / restart, the MMU unit performs a self-test. It checks if the main control board behaves correctly and if all the signals from the microcontroller get to the TMC motor drivers on the control board properly. If the MMU unit suspects the communication with the Selector motor TMC driver might not behave correctly, this error is shown.
- Make sure your MMU3 unit is correctly assembled with the PD-board addon attached.
- Verify all the cables are in good condition and the connections are stable. Do not move, connect or disconnect the cables while the printer is on!