Col,
When its in full SI screen check the range by looking at the top corner of the SI screen. The dark area is the water column, so I'm thinking you either have the range in so close that it isnt hitting anything, or you are in water that is too deep for the unit.
To change the SI range. Go to SI screen hit menu button once. Scroll down to the range, edit & exit button until menus close.
Qs-
Is the unit working in normal 2D mode? Is it showing bottom? If so I think the Xducer is ok.
Is the screen color ok in normal 2D view &/or Chartplot screens? If so the backlighting & pixels are ok so it aint that.
If all else fails, make sure you have the range set properly & run it along some known structure like under a bridge pylon, snags or rocks & see if you get some returns.
These SI units can give some fantastic images. Im always amazed at some of the things I find.
Cheers,
fitz..