I tried all frequencies.
I sail in very shallow water (avg depth 13 feet).
The original unit was malfunctioning. It was replaced by Raymarine under warranty. The computer is in the diplay unit which was where the problem originated. Raymarine is aware of this problem with some of their units.
Contact Raymarine and they will suggest that you try a replacement. This is how I found my problem last year. Once replaced there was no longer any problem.
Last Saturday I had the keel offset set to the depth from my hull to bottom. It worked reliably right to the point where it said 0.4 feet when I realized my offset was off by 0.4 feet. As it was a rising tide and the bottom is sand and mud we simply heeled the boat over and kept on going til we were out of the channel. We dragged the keel pretty much the whole way. The unit worked properly the entire time. The depth was about 3.5 feet.
Replace the instrument not the depth transducer and your problem will be solved under warranty.
Best of luck