The RED LED light is an indicator of either NO 12 volt fuse in the adjacent slot or the fuse is BLOWN. AS you found out, with a LOOSE fuse, it acted as if NO fuse. When a 12 volt circuit has a LOAD on it and the fuse is Bad or missing, the corresponding RED led light comes ON to let you know which fuse needs fixed. Not all 12 volt panels have this Red led light fault system. The only drawback to this system is, IF the Blown circuit has NO load on it, the Red led light will not light. So always manually check all fuses when you have a 12 volt outage. Doug