A circuit tester, commonly called a test light, is a quick tool used to diagnose electrical issues. It consists of a sharp metal probe, an internal bulb or LED indicator, and a long ground wire with an alligator clip. You connect the alligator clip to a solid metal ground point on the vehicle's frame or battery negative terminal. When you touch the probe tip to a hot electrical terminal or wire, the bulb lights up, showing that voltage is present.
Test lights are ideal for checking fuses, relay sockets, and headlight connectors. By touching the probe to the tiny metal test ports on top of a fuse, you can instantly see if power is passing through without removing the fuse. While fast and convenient, classic incandescent test lights draw current from the circuit they test. This can damage delicate computer circuits (ECUs), so a high-impedance digital test light should be used on modern computerized cars.