A red LOS (Loss of Signal) light on a fiber modem indicates no optical signal reception, often due to fiber cable damage or loose connections. When it's green and steady, everything is fine. Unplugged and plugged in the power to reboot but still red. Just wanted to know if anyone knows or have fixed this problem without a tech guy. :. This is displaying a solid red LOS light - which I understand to stand for Loss of Service. Should I just turn it off and on? Or does the LOS light mean there's a fault outside with the BT/Openreach network? Solved! Go to. If you find that the Optical/Config/PON Light on your Fibre ONT (Optical Network Terminal) box is flashing, has gone off, or has gone red, this indicates there may be an issue with the fibre connection coming into your property. What Can I Do? First, please check that the optical cable which comes. The router red light is flashing, I've tried a reset, plugging / unplugging cables. Firstly call it in on the “faults” line - on 150 (VM Phone - free) or 0345 454 1111 (any other phone - national rate).
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