I had the same problem, measured the current over the indicatorlight, disconnected all wires (negativ and positiv) of all equipment in contact with salt/waterpumps. I could not find the fault and had to leave the boat for several weeks. When I returned the test worked fine - no Indikator light. This was 18 months ago-still no problems.
After this I check it daily, sign o.k.in the logbook. Minimum test before and after any work at the electric System.
Once I had a light on the tester, when I made a HDMI connection between the TV and the Furuno Navnet.
