NL-4 SNIFFER/SENDER F.A.Q
Typically this occurs when the batteries are put in backwards. Reversing its position will return it to all green.
While the NL4 Sniffer/Sender was not designed to test NL2 cables, you can use the tester on 2 wire cables. You will get a readout of "A=Green," "B=Red," "C=Green," and "D=Green." Alternativly you can test two NL2 cables using a "Y" adapter.