Canon MegaTank PIXMA G6020 shows a paper jam or repeatedly jams

The checks in more detail

Find the cause. Paper that is curled, damp or of the wrong weight jams, as does a stack that is overloaded or loaded crookedly. Fan the stack, square it and load it against the guides, below the mark. The G6020 takes plain paper in the rear tray and the cassette, but thick or glossy paper needs the rear tray and the right type setting. Mixed sizes in a tray cause skew. Check that the paper support and the output tray extension are open for the size you use, since paper falling off the tray can jam the exit. The rollers collect dust. Canon's manual describes cleaning the feed rollers using the printer's roller cleaning function, found under Maintenance,

which feeds the rollers without paper to clean them, and wiping the rollers with a damp cloth when the printer is off. A recurring jam in the same place may mean an obstruction or a worn roller. If the printer shows the jam message with an empty path, switch it off at the wall for a minute, since a sensor may be stuck. Keep the paper in its packet in a dry place. Contact Canon with the model number and the support code if jams continue after cleaning with good paper. The G6020 is a Canon MegaTank with a rear paper tray and a cassette, and paper jams in it are mostly about the paper path rather than the ink tanks.

Open the rear cover and look for torn corners left behind; a half-sheet stuck deep can be reached with tweezers after you switch off and unplug the printer. Fan the paper before loading, do not mix sizes and slide the guides to touch the stack without bending it. Paper that has absorbed moisture curls and feeds badly, so use sheets from a sealed pack. Check the paper thickness against the limits in the manual, and select the matching paper type on the printer's screen. The feed roller can pick up paper dust and slip, so wipe it lightly with a barely damp lint-free cloth. After clearing a jam, press OK on the printer to resume.