Check the inline fuse located on the main power supply to the unit. Visually inspect it and if it's blown, replace it with a 1 amp, normal blow fuse. To be absolutely sure the fuse is good, use an OHM Meter to check it.
Clean all corrosion from the fuse housing, and replace the fuse holder assembly if necessary.
Check the power cable connection.
Red wire to positive, Black to negative or ground.
Ensure that the power source is supplying a steady 12-volt power supply using a volt-meter, multi-meter or test light.
Remove the depth finder from the boat and connect it directly to an external 12-volt power source.