Samsung SCX-8240NA-CAS - Manual (Page 198)

Troubleshooting_ 198
15. Locate the LSU window cleaning hole and insert the LSU window
cleaning stick. Slowly push and pull the LSU window cleaning stick 4 or
5 times to clean the LSU window.
16. Insert the LSU window cleaning stick back into its place.
17. Insert the waste toner container until it locks in place.
18. Close the front door. Ensure that the door is securely closed.
If the front door is not completely closed, the machine will not
operate.