2012 Isx 2350 475 with codes 4726 and 0559

[Hermjeji]

There is a check valve in the fuel pump head. If your plunger is not "popping" back up, your check valve has failed and you will need a new head. The kit I believe comes with new springs, and plungers all assembled. This video explains it well: https://www.youtube.com/watch?v=wKLV4gAUl6o

There's also the rebuild video here: https://www.youtube.com/watch?v=-Gu5Eyx2SWw&t=91s