I would start looking at the wiring between the power cable for the unit and the battery it is powered from. Any loose or corroded connection can rob the unit of the power it needs to work properly – which can include how its sonar works. While you may read 12.4 volts on a non-loaded (the unit is not connected and powered on) connection the unit may not be able to get the current it needs. Be sure to check the fuse, fuse holder and battery terminal connections. I recommend the unit be on its own set of wires (16 AWG multi-stranded copper) and fuse for best results. Solder and seal all electrical connections.