Designed to quickly test the condition of your relay out of the vehicle. Fully self-contained, no need to clamp cables onto the vehicles battery. Accurately check the relay coil for circuit breaks, and switch contacts. Test results provide good/bad indication; green LED pass, red LED fail. Used to test the most common 12 volt 4 and 5 pin relays on today vehicles. Compact design, nothing additional needed, and super easy to use. 4 AA batteries included.