My XLR/DMX Sniffer/Sender has unlit LEDs when I plug into a known good cable. What is wrong?

This is most likely a low battery issue. Senders operate by sending power to the Sniffer. If the battery is low, it will not have enough voltage to properly test, some LEDs may appear unlit.

You can use a multimeter to check and see if it is still running at 12v. If not, a fresh battery will get you back up and running.

The other way you can test is to plug the Sniffer into a mixer and turn on phantom power. If all lights come up green, it's for sure the battery.

In case you're not aware, the Sender end has a twist cap to turn on and off the Sender to save battery when not in use.