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After a startup or 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 that the communication with the pulley motor TMC driver might not behave correctly, this error is shown.
Access the MMU control board, and inspect it visually to spot any incorrect assembly or damage on its surface, on the cables, and attempt reseating the cables to ensure their connection. In particular, make sure that the PD-board add-on is attached to the control board.
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If the issue is not solved with these checks, please contact our customer support.