Many years ago I have similar problem with an Epson LQ1170 Dot Matrix printer, i.e. a Rat somehow found its way inside the printer and peed on its board. Anyway, the long and the short of it Epson refused warranty on similar grounds stated above, however, our supplier was generous enough to foot half of the total bill for replacement board in the end.
So, I would say you are probably better off buying a new graphic card instead of wasting your time going after warranty claim which can't be honored.