Clean the toner density sensor when the following message appears on the control panel.
"Toner Den.Sensor cleaning req."
Pull up Top Cover Open Lever to open the top cover.
Lift out the cyan print cartridge.
Slide the toner density sensor lever once to the left.
Slide the lever only once.
To reattach the cyan print cartridge removed in step 2. Push it all the way in carefully.
Close the top cover with both hands carefully. Be careful not to trap your fingers.