This is not your model but it may be related to your issue
The 5100 error in the i850 is carriage related, specifically it points to the
code strip, carriage assembly or logic board.
If it is the logic board or carriage assembly then it is probably time to
replace the printer unfortunately.
Go to the canon website (listed below)-under Tech Support - click on Technical FAQ's (this will pop up another small window)
then click on Search by Key Word
next - type in error 5100 and click on the circle next to - Include all words and hit Search
next - click on the link with your answer "how to.... "
this should show the answer