Red LOS light on a fibre router: causes and what to do

By The SpeedFibre team, updated . 3 min read.

Red LOS light on a fibre router: causes and what to do
In short LOS (Loss Of Signal) means the router no longer receives light through the fibre. Check that the optical cord is clipped in at both ends and is neither bent nor crushed, then restart. If the light stays red, the fibre is cut upstream: only the provider can fix it.
Contents
  1. What does LOS mean?
  2. Checks you can do
  3. If the light stays red
  4. Blinking LOS: weak signal
  5. Once it is fixed

The LOS light (sometimes "Alarm", an unlit "PON" or a red "Fibre" light depending on the model) is the clearest sign of a fibre line problem. As FTTH technicians, it is the alarm we see most often in the field. Here is how to read it and what you can safely check.

What does LOS mean?

LOS = Loss Of Signal. The ONT or router no longer receives the light sent by the provider's equipment. Without light no data passes: internet, phone and TV are all down.

Light statusUsual meaning
LOS off, PON solid greenOptical signal fine, line working
LOS solid redNo optical signal received
LOS blinking redSignal too weak or unstable
PON blinkingRegistering with the network

The exact meaning can vary by manufacturer: also check your router's guide.

Checks you can do

  1. Is the optical cord plugged in properly? Make sure it is fully clipped into the wall socket and the router. You should hear a small click.
  2. Is it bent, crushed or pulled? Follow the cable: a door, a piece of furniture or a tight bend can block the light. Fibre does not tolerate sharp angles.
  3. Has someone moved the router or cleaned around it? One of the most common causes.
  4. Restart the router by unplugging it for 30 seconds.
  5. Do not clean or open the connectors yourself, and never look into a connector or the end of a fibre: the laser is invisible but dangerous to the eyes.

Measure your connection now. Start the test

If the light stays red

The cut is probably upstream: damaged cable in the building or street, roadworks, an open or damaged distribution box, or a problem on the provider's equipment. In that case:

  • Contact customer service and say "solid red LOS light" (or blinking): this wording routes you straight to a technical intervention.
  • Say since when, whether there have been works in the street or building, and whether neighbours have the same problem.
  • Keep the cord connected and the router on to allow remote tests.

Blinking LOS: weak signal

A blinking LOS or repeated drop-outs with the light turning red often indicate excessive attenuation: dirty or damaged connector, degraded splice, partly crushed cable. Speed can also become unstable. This is also a case for the provider, who can measure the received signal level. To understand the light's path, read how FTTH fibre works.

Once it is fixed

When the light is back to normal, run a fibre speed test over a cable to check that speed and ping are back to normal. If drop-outs repeat, keep a log as explained in frequent internet drop-outs.

#WiFiDiagnosis#Ping#Troubleshooting

Sources and further reading

Frequently asked questions

Can a red LOS light come from my router?

Possible but rare. Most often the cause is the optical cord or a cut upstream. The technician checks the signal at the socket before replacing the router.

Can I replace the optical cord myself?

If your provider gives you a compatible replacement cord, yes. Do not use a cord with a different connector type.

How long does it take to repair a cut fibre?

It depends where the cut is: a simple reconnection can be quick, a cable cut in the street takes more work.

The SpeedFibre team Guides written and reviewed by a telecom engineer specialised in fibre optics (FTTH), with more than 8 years of hands-on network maintenance and measurement. Editorial policy.

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