The NL-4 Sniffer/Sender field unit is an all-analog remote end cable tester that tests for good connections as well as 16 different issues in speaker cables. The sender unit uses a car alarm battery with long-lasting life to power the unit. Four LED lights on the sniffer unit instantly display results, giving you a quick-and-easy way to check whether your line is good. If all 4 lights are green, you're good to go. The NL-4 Sniffer/Sender is made with the same high quality that allows us to keep a lifetime Happy Human Warranty on all our products. A durable anodized aluminum tube protects the all-analog testing units. There are no fragile moving parts or seams, making these some of the most indestructible testers on the market.