Canon iPF8400S User Manual page 822

Replacing the Printhead
3
If the ventilation duct (a) is dirty, use a damp cloth
that you have wrung out completely to wipe it clean.
If the Carriage is obscuring the ventilation duct, move
the Carriage manually before cleaning the duct.
4
Pull up the Carriage Cover to open it completely.
5
Pull the Printhead Fixer Lever forward all the way to
open it completely.
822
iPF8400S
User's Guide