if you open the door by the fuser you will see 2 square pieces of glass try cleaning those and turning the machine off and on.. if that does not fix the problem you need a tech
e3-40 usually appears when the copy density sensor is dirty. it is located at the bottom of the drum unit. you may pullout the drum unit from the machine and disassemble it to clean the copy density sensor board. it is a small PCB that has two proximity sensors.
pls. keep in touch if you still need an assistance
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