I had the same problem with my MX340. I called canon and it turns out that something have falling in the paper tray. tilt it to the back and stick your finger in there and try to move the rollers or shake and tap it a little till it falls out. I had a little nail in there. it's working find now.
It is hard to reply without knowing what your actual attempted fixes were. I have the online Manual and the main solutions, if the error message was on the LCD, were focused on the Fine cartridges..making sure they were installed properly and if you might possibly be running out of ink.
Press the "Menu" button on the MX340 until "Device Settings" appears on the LCD display.
Press the left or right arrow keys to select "Reset Settings," and then press "OK" to load the Reset Settings menu.
Press the left or right arrow keys to select "All data," and then press "OK" to reset data on the MX340 to factory default settings.
I had the exact same problem. The Alarm light was always on and the screen said "Document in ADF. When I pressed "OK" it would read "Eject the document". Even after I placed paper in the ADF and the paper would feed the document through it, the error and the Alarm light was still there. I found the solution, Lift the cover to expose the flat bed scanner glass. Under the cover, there is a long white bar with two white tabs perpendicular to the white bar. One of the tabs was retracted. Pull on the tab until it is fully extended. The alarm turns off and there is no longer the error. Simple solution to a damn annoying issue.
I had the same problem...after trying everything suggested on the forum. I had enough and decided to disassemble the printer. I removed the CAD and disconnected the wires and it solved the problem. No more paper jam error code on the screen. This probably includes that the CAD won't work anymore but I never used this anyway. So now I can print and scan like I used to.
Removing the covers is not the answer. Most jams on rear feed printers/faxes are due to some object falling in the sheet feeder. When you pull the paper back out (Very slowly) of the sheet feeder after it has jammed , you will see a nick in the paper. This is because the paper is hitting something and cant get past it. put the paper back in the sheet feeder and the nick will indicate where the object is. Find a large paper clip and unwind it and bend the top of it to 90 degrees. use this to hook the object out and then tip the printer upside down and watch it fall out. Good luck