A tester for 12 V 7-pin trailer wiring, with a plug on one end and a socket on the other, so it checks both the car's socket and the trailer's plug. Seven LEDs show which circuits are live.
To test the trailer, plug it into the tester and press the button on the side: every LED should light. To test the car, plug the tester into the car's socket and work the lights, indicators and brakes in turn. Each one should light its own LED.
The pins follow ISO 1724: 1 left indicator (yellow), 2 rear fog light or permanent 12 V (blue), 3 earth (white), 4 right indicator (green), 5 right tail light (brown), 6 brake lights (red), 7 left tail light (black). It doesn't test 13-pin sockets or 24 V wiring.
