This LED testing device checks all the lighting functions important in traffic on the vehicle trailer sockets & trailer plugs. The LED indicator shows the defective cable colour in each test. It tests all 7-pin sockets & plugs &, using the two adapters supplied, also all 13-pin sockets & plugs for their 7-pin functionality. Both adapters can also be used between a 13-pin tow vehicle & a 7-pin trailer & vice versa. Power supplied by a 9 V PP3 battery (sold separately).