Red LOS light on your fiber: what to check without touching the external network

What does the LOS light mean?

On many optical devices, LOS stands for “Loss of Signal,” meaning the device has detected no signal or a signal loss. However, the indicator’s color, whether it flashes, and its exact meaning depend on the model and its documentation. A red light alone cannot establish whether the cause is inside the home, in the equipment, or on the provider’s network.

Check the indicator's name and color, and whether it stays lit or blinks. If possible, note the status of the other indicators and when the change began. Photograph the panel without including the label showing the password, serial number, or account information.

Check only items that are safe and accessible

• Check that the router's power cable is plugged in and that the device is switched on.
• Without moving the fiber-optic cable, check whether it has a visible bend, is being crushed, or is under unusual tension.
• Check your model’s guide and your operator’s messages for the appropriate procedure.
• Note whether the connection is down on all devices or whether only Wi-Fi is affected.

Never look into the end of an optical connector, and do not unplug the fibre to “clean” it or reset the signal. Do not touch outdoor enclosures, cabinets, or the operator’s equipment.

Restart only as instructed

A simple restart may be recommended in some providers' instructions, but it should not be confused with a factory reset. A factory reset can erase settings and is not a first-line troubleshooting step. Follow the procedure for your equipment or ask your provider for advice.

After following the permitted procedure, check whether the indicator lights change and whether service returns. If the LOS light remains on or the connection is still down, keep your notes and contact your provider about issues involving its network. An in-home diagnosis can check accessible equipment inside the home, but it does not replace an external service visit.

Describe the problem clearly

If you know it, provide the model, the names and status of the indicator lights, when the outage began, which devices are affected, and which checks you have already performed. This information is more useful than simply saying, “The internet has stopped working.” For further observations to make, see our guide to distinguishing a Wi-Fi issue from a fibre problem.