If the LOS light on your fiber router or ONT is blinking red, it usually means Loss Of Signal. This guide explains the likely causes, the checks you can do at home, and when the issue needs technician support. When it's green and steady, everything is fine. Sometimes it may be due to a problem with your internet service provider, although you could also be experiencing this issue due to improper configuration of your router, a poorly connected cable, etc. Fast pulsing red lights often indicate a factory reset is in progress.