Canon PIXMA TR4520 (TR4500 series) support code 6000

LCD display says "support code 6000". inspired by https://youtu.be/EI63iQmJza4, i opened it up looking for a paper jam.

although i unscrewed every visible screw, only 4 were necessary to access the print head and carriage underneath the scanner bed (platen).  however, one screw is inside the U-turn of the scanner ADF (underneath the controls), which feels loose, like it can be opened, but it isn't obvious how.  how: there is a spring-loaded latch next to the scanner bed: use a narrow tool, e.g., screwdriver, to retract the latch, then the ADF U-turn will open, revealing one of screws to open the printer.  presumably the ADF U-turn is accessible to clear paper jams.

the spring-loaded latch can itself be removed and disassembled.  it's a little bit tricky to put the spring back to reassemble.

after opening, there are ribbon cables between the two parts.  i left those attached.

i moved the carriage back and forth and moved the gears but didn't see any obstructions or jammed paper bits.  reassembled the printer, and error message no longer occured.  happy to achieve victory, but disappointed not to be able to tell what was wrong.

the underside has parts that can be removed by unscrewing.  one of the parts is nontrivial to put back.  there is a wheel or gear with a notch, and things only fit if the gear is at the correct angle to accept a piece. there are two notches, each differently shaped.  i picked one randomly and it worked, and did not investigate fitting the piece into the other notch.

there is a small arm that extends when the gear is at the correct angle.

when the underside piece was not in the notch and printer reassembled, there was some error, but i did not record it.