What do the lights on my fibre box mean?
Your ONT tells you exactly where the fault is, if you know how to read it. Here's every light, every state, and what to do.
Normal. Counter-intuitively, LOS being dark is the healthy state — it is an alarm light.
Loss of optical signal — the ONT isn’t seeing light from the network. Commonly a damaged or disconnected fibre, a bent or unseated cable, or a fault upstream.
What to do: Check the thin fibre cable is fully seated and not kinked or crushed by furniture. Power-cycle once. If LOS stays red, it is a line fault — report it and say "the ONT is showing LOS", which usually gets it escalated to the network operator rather than looped through ISP-level scripts.
Why the lights matter more than the symptom
“My internet is down” could be a dozen things. “My ONT is showing a red LOS light” is one thing: the fibre isn’t delivering light to your house. That single distinction decides whether you’re about to spend twenty minutes resetting a router for no reason, or reporting a line fault that only the network operator can fix.
It also changes how your fault is handled. Quoting the light state tends to get a ticket escalated to the network operator rather than looped through ISP-level scripts — you’ve already done the diagnostic step they were going to walk you through.
A caveat worth reading
Exact labels, colours and flash patterns vary between manufacturers — Huawei, Nokia and ZTE units all differ, and firmware changes things too. Some ONTs have no LOS light at all and signal the same condition through PON. What’s above is the common GPON convention, and it’s right far more often than not, but check the label on your specific unit before treating any of it as gospel. If your ONT is combined with your router, you may also see Wi-Fi and Internet lights that a standalone ONT wouldn’t have.
Not sure what to do with what you’ve found? The fibre troubleshooter walks from the lights through to a likely cause, and WiFi not working — fixes by provider covers what to do per ISP.
ONT or router — which box is which?
The ONT is where the thin fibre cable terminates. It’s usually a small white or black box mounted on a wall, often in a cupboard or near where the cable enters, and it has the PON and LOS lights. The router is what broadcasts your Wi-Fi and is connected to the ONT by an ordinary network cable. Some installs combine both into one unit.
The distinction matters because they fail differently: an ONT fault is nearly always the network’s problem, while a router fault is nearly always yours. If both look healthy but nothing loads, the issue is usually authentication or DNS — the troubleshooter separates those.
Line faults that keep coming back?
See which other fibre networks reach your address — there’s often more than one option in a street.