Canon iR C6800 Series Service Manual page 1200

Chapter 15
5) Select the time, and press the OK key.
6) While referring to the index, move the sensor toward the paper from the position farthest away from the paper;
then, fix it in place where the indication turns 'OK'.
F-15-22
Memo : If the indication does not change to 'ON' even when the tray full sensor has been brought closest to the paper,
add a sheet of paper until it does.
[1]
F-15-23
7) Move the stack of paper to the point of detection of the other tray full sensor.
8) Perform step 4) once again to adjust the position of the sensor.
9) When done, press the Stop key.
15-136