Dell 5100cn Color Laser Printer - Manual (Page 209)

3. Replace the printer's tray into the printer.
Cleaning the Automatic Density Control (CTD) Sensor
CAUTION: Before performing any of the following procedures, read and follow the safety instructions in
your Owner's Manual.
CAUTION: Do not touch the edge of the plate at the upper side of the transfer roller because it is sharp.
When the printer's automatic density control (CTD) sensor is dirty, follow the procedure below to clean it.
1. Turn off the printer.
2. Push the front latch up and fully open the front cover.
3. Grip the tabs at both ends of the transfer roller and gently lift it out of the printer.