Lowrance fish finders send an electrical impulse turned into a sound wave through water. When the sound wave hits something, the object reflects the sound back to the fish finder's transducer. This process is called SONAR (Sound Navigation and Ranging.) The transducer sends the signal back to the Lowrance fish finder that converts the data into a distance calculation. One of the main problems with fish finders is interpreting the objects that the sound wave hits---hopefully the fish. Learning how to read the raw SONAR data solves most problems.
Change the batteries on the fish finder if the fish finder uses batteries and the display appears dead. Verify that the Lowrance fish finder is connected to the vessel's power supply if the device doesn't use batteries. Look for illuminated lights or a display image. Clean the power cable pins with an emery boar. They can become oxidised in the salt air.
Switch the Lowrance fish finder on. The default mode when you turn on the device is "Fish ID." Fish ID mode is a graphical interpretation of the SONAR.
Travel over a suspected fish area and take a look at the screen. Fish are represented on the screen by fish icons. These are graphical representations of the fish. They are not the actual SONAR readings and are an interpretation by the device. Fish ID can provide false readings. Bubbles and tree limbs can be misidentified by the fish finder as fish in Fish ID mode.
Learn to read the actual SONAR images. Turn off the Fish ID mode with the mode button. The graphical fish icons will disappear. You will see arches where there are fish. Bubbles and surface clutter that showed as fish icons in Fish ID mode will now be shown as pixilated dots. The arches are the true reflected SONAR images.