Use this tester to check if 12v power from the tow vehicle is reaching the trailer's tow plug. This tester also indicates if the vehicle's tow circuit is properly configured. Easy to use: Plug tester into tow vehicle's towing plug, operate vehicle's turn signals, lights, and brakes. When power is present, the tester's green LED will glow. Tips: if LEDs are dim or flicker, check for loose or corroded ground wire. To test the tester, use 12v batter charger to determine if LEDs power is up.