The gear especially detects the wires with bad contact or aging wires. Before test, connect the alligator clip of black test pen to the ground, turn the knob switch to the test lamp & multimeter voltage gear "V--", use the feeler to hook the detected rubber cover wire, press down trigger, and head the probe against the front detected rubber cover wire. If the front end of the detected wire is 12V (normal voltage), and then measure the voltage of back end of the wire, "if LCD displays the voltage less than 10V" indicates that the back end of the wire has bad contact or is aged.