Part of towing responsibly is making sure your trailer lights are working properly. Whether you have just installed a new custom wiring harness or are trying to locate the cause of an electrical problem, CURT connector testers are great for testing tail lights, turn signals, brake lights, ground connections and other electrical components on your vehicle. A connector tester works by plugging into the connector socket on your vehicle and responding to the various electrical signals produced by pushing the brake pedal, signaling a turn, turning on the running lights and so on. When the socket is getting proper power, the tester's various LEDs will light up corresponding to each wire in the connector. Testing your vehicle-side socket is essential after installing a new custom vehicle-to-trailer wiring harness and is highly recommended every time you hitch up to tow. Also, if you are experiencing electrical problems with your trailer, a CURT tester will be able to tell you whether the problem is rooted in your vehicle's electrical system or if your trailer needs some rewiring. All CURT connector testers come with LED lights to be bright and long-lasting. Wire testers are available for 4-way flat, 5-way flat, 6-way round and 7-way RV blade connectors. This particular electrical tester fits a 4-way flat socket and comes with quality plastic construction.