Re: Help - Canon IR 3100CN Error Code E000020-0081
Open the side door fully. From the front, take out the small black screw holding the black lever above the drum and rack the lever 45 degrees clockwise. Take out the 2 silver screws holding the drum in place. Remove the drum by pulling it out towards you. Inside above where the drum goes you should see a cream coloured plastic unit about 2 -3" long and that will have what looks like a small black window in the middle of it which will probably be covered in toner. Clean it ! That will probably fix it for the time being, until it gets dirty again. As previously stated you might need a new drum though. Check for a line along the drum, often referred to as a "burn mark". If the sensor is getting dirty repeatedly then new developer assemblies could be required as well as they leak toner when the starter gets old and worn.
It could be an easy quick fix but to do it properly will be expensive. Better to get a new copier !