Canon PIXMA MP780 paper jam eject slot error message

Canon PIXMA MP 780 paper jam error message at ejection slot remains after paper jam removed.

INITIAL PROBLEM
paper jam problem began when a dangling telephone wire got caught in paper intake. it was removed very gently. no error message.
Tried to print, got error message at ejection slot
turned machine off and on. after being turned on, a sheet of paper will pass through the machine but paper jam message continues.

WHAT I HAVE TRIED, FOLLOWING EARLIER FIXYA SUGGESTIONS
since then. I have.
1. turned printer upside down
2. gently cleaned out paper dust on either side of the paper rollers
3. checked that paper roller is still rough
4. tried to change to feed from lower cassette. will not accept feed switch command
5. removed and reinstalled print head (which was replaced last month)
PROBLEM REMAINS.
turned machine off and on. after being turned on, a sheet of paper will pass through the machine but paper jam message continues.

QUESTION: WHAT TO DO?

Background: we have in past two months cleaned the WASTE TANK following the instructions here and reprogrammed it and, after the print head refused to print from the smaller of the black cartridges, replaced the print head. We get a flashing light when printer is turned on and a small grind, but then the light disappears.