Why is my Canon TS8150 not printing some items of text

My Canon TS8150 printer seems to print ok, except that it fails to print some items of text. When I print a test page it just prints the blue header but not the rest of the text, and if I create a Word document it misses out some of the heading text. I suspect the firmware in the printer is corrupted, is there any way of reinstalling the firmware? If I try a firmware update it fails because it says the printer is already up to date.