Has the yellow sucked up some magenta? Common with the shared head. Maybe time for a new cap. Also be sure machine is level. I heard if one cartridge is full and the other near empty it can cause the ink to be sucked back. Do a regular clean until it prints clear, or just print big bars of yellow until it comes clean.
Cap tops, clogged/dirty pump lines or pumps going bad can all cause this. I think dampers can possibly cause it also but from my experience it is usually a from the cap tops down issue.