Full fibre / Equipment guide
What Do the Lights on a Fibre ONT Mean?
A quick way to tell whether the problem is power, the incoming fibre, the cable to your router or something further along the service.
By Hasnaat Mahmood · Published: 29 Sept 2026 · Updated: 29 Sept 2026The short answer
Power on, PON steady green, LOS off and an active Port/LAN light usually mean the ONT has power, sees the fibre network and is linked to your router. A red LOS or Alarm light, or a PON light that stays off or flashing, points towards a fibre or network connection issue. If PON looks normal but Port/LAN is off, check the Ethernet cable and router. Labels and colours vary by model, so use your provider’s guide for the exact unit.
- POWER
- PON
- LOS
- LAN
- POWEROn · box has power
- PONSteady · fibre linked
- LOSOff · no signal warning
- LANOn/blinking · router linked
This pattern describes many Openreach-style ONTs, not every fibre box. Some show Optical instead of PON, or Broadband, Service, Alarm and Ethernet instead.
01 / Read the box
What each ONT light tells you
The optical network terminal, or ONT, is the powered box where full fibre enters your home. It connects to the router by Ethernet. The lights narrow down which part of that route needs checking; they do not always identify the precise cause.
- Power POWER / PWR
- Steady green: the unit is powered. Off: check its power switch, adaptor and socket. A light flashing during startup or an update can be normal on some models.
- Fibre link PON / OPTICAL / BROADBAND
- Steady green: the fibre link is established on many units. Flashing: the ONT may be trying to connect or register; allow it to settle after startup. Off: the connection is not established. Check the matching model guide because Optical and Broadband indicators are not identical on every device.
- Signal warning LOS / ALARM
- Off: normally the expected state when the fibre link is healthy. Red: often means the optical link is down or the unit cannot connect. A flashing red light can mean something different during a firmware task on certain models; consult the exact provider guide before interpreting it.
- Router link PORT / LAN / DATA / ETHERNET
- On or blinking: usually an Ethernet link, with blinking often showing traffic. Off: check the cable from the ONT to the router’s WAN/Internet port and confirm the router has power. Some multiport units use only the provisioned port.
- Phone service TEL / PHONE
- A lit telephone indicator may show an active voice port where that ONT provides the phone service. Off is not automatically a fault: many customers have no ONT phone service, and some digital phones connect through the router instead.
Do not judge every box by the same colour. Zen’s CityFibre guidance, for example, says the Data light on one white Nokia ONT can be green, yellow, white or blue according to the Ethernet link speed. Identify the network and model before treating an unfamiliar colour as a fault.
02 / Match the symptom
Which light should you check first?
All lights off: start with power
Make sure the ONT’s own adaptor is connected, the wall socket is switched on and any ONT power button is on. If a known working socket still leaves it dark, report that to your broadband provider.
LOS red or PON never settles: check service status
Look for a reported outage. Check whether the visible fibre lead is obviously trapped, sharply bent or loose, without pulling it out. After a normal power restart, give the ONT several minutes. If the warning persists, report the light pattern to your provider.
PON steady, Port/LAN off: follow the Ethernet cable
Check that it is seated at the ONT and at the router’s WAN/Internet socket, and that the router is on. If available, try another Ethernet cable. The optical connection may be fine while this local link is missing.
ONT lights normal, internet still down: test beyond the ONT
Restart the router, check its internet status and try a second device. Working ONT lights do not prove that the provider’s internet service, router settings or Wi-Fi are working. Check your provider’s outage page and report the fault if it continues.
Keep fibre handling minimal. Do not stare into the optical connector, unplug the fibre lead for routine troubleshooting or press a recessed reset button unless your provider directs you for your model. A power restart is different from a factory reset.
03 / Next step
What to tell your provider
Take a photo of the ONT and note the exact labels, colours and whether each light is steady, blinking or off. Tell your provider when the fault began, whether every device is affected, what the router shows and whether you have tried a power restart or another Ethernet cable. Your broadband provider is the right contact even if another company owns the fibre network.
If your phone service also runs over broadband, an ONT without power can affect calls. Households that rely on digital landlines should read our digital landline power-cut guide for the separate backup-power question.
The useful rule
Read the ONT in order: power, incoming fibre, then the Ethernet link to the router. A steady PON with LOS off is reassuring on many Openreach boxes; it does not guarantee that Wi-Fi or your internet service is working. Use the exact model guide if your labels or colours differ.
FAQs
Should the PON light be solid or flashing?
On many Openreach-style ONTs, PON should settle to solid green when the fibre connection is established. Flashing can occur while the box is connecting or being verified. If it keeps flashing and you have no service, contact your provider.
What does a red LOS light mean?
On common ONTs, a red LOS light indicates a problem with the incoming optical link or connection to the network. Check for a known outage and report a persistent red light to your provider. Some models use different warning patterns, so check your exact guide.
Why is the LAN or Port light off when PON is green?
PON can show a working fibre link while the Ethernet connection to the router is missing. Check that the router is powered and the cable is secure in the ONT port and the router’s WAN/Internet port. Try another cable if you have one.
Can the ONT lights look normal while the internet is down?
Yes. The ONT may have a working fibre and Ethernet link while the router, provider service, Wi-Fi or a particular device has a problem. Test another device, check the router and your provider’s service status, then report a continuing fault.