Furuno GP-3500F Operator's Manual page 223

If...
picture does not move though marks and
characters appear
zero line does not appear though the picture
appears
picture sensitivity is too low
the depth indication is not displayed
noise or interference shows on the display
data is wrong though the water temperature
graph appears.
12. MAINTENANCE & TROUBLESHOOTING
Sounder troubleshooting
● check that "7. STOP" is not selected of the
PICTURE ADVANCE on the ECHO SOUNDER
SETUP menu.
● check for loosened transducer connector.
● the picture is shifted. Confirm shift setting.
● check gain setting, if using manual operation.
● marine life or air bubbles may be clinging to
transducer face.
● bottom may be too soft to return a suitable
echo.
● adjust gain and range to display the bottom
echo in reddish brown, if you are using the
manual operation.
● If no noise appears after turning the engine off,
the engine interference is suspected. Call your
dealer.
● check the ground.
● other video sounder of the same frequency as
yours may be operating near you.
● check that sensor cable is tightly fastened.
Then...
12-5