Canon imagePRESS C800 Series Troubleshooting Manual page 143

Chapter 1
b) Dustproof Glass (Bk)
b-1) Pull out the Dustproof Glass [1], and clean both sides of the Dustproof Glass with lint-free paper [2] moistened with alcohol.
[Caution] When installing the Dustproof Glass, insert it into the mounting hole [1] with the mark "UP" [A] upward.
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