I have a YM2002d, I will try to look at it and see what I can figure out. Memory says that blank (no icon) on your photo of the YM226 is also blank on mine and does not light with the “test” button pressed.
I believe they are electric, oil pressure and coolant. Mine has a temp gauge so I seldom look for that light, but watch the gauge. Unfortunately the gauge is electrical, so that is a possible issue as well.
I have not gotten to the tractor yet but found this info in a operators manual on 02 series YM’s. It goes down to 1702 and up to the 2002 models.
View attachment 708593
Hope it helps. When I get to the tractor, I will try to get a photo if the icons on the tractor warning lights.