HP Color LaserJet Enterprise M751n Laser Printer - Manual (Page 85)

6. Close the left door.
NOTE: If the toner-collection unit is not installed
correctly, the left door does not close completely.
To recycle the used toner-collection unit, follow the
instructions included with the new toner-collection
unit.
7. Close the front door.
ENWW Replace the toner-collection unit 75