MMU not responding #04401 (MMU)

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What happened?

The printer is trying to initiate communication with the MMU unit, but there is no response.

How to fix it?

Check the following:

• Make sure the MMU unit is present on the printer. If not, either connect the MMU unit back or turn the MMU off either by selecting Disable MMU or in the printer’s menu.

• Verify all the cables are connected properly. Do not move, connect or disconnect the cables while the printer is on!
• Verify all the cables are in good condition and the connections are stable. 
• Make sure your MMU3 unit is correctly assembled with the PD-board addon attached.
• Make sure you are using the correct up-to-date firmware for both the printer and the MMU unit.