Power or ONT hardware
Use a known-good wall socket and record whether the ONT repeatedly restarts. Do not substitute an unapproved power adapter.
Full fibre ONT light guide
Full Fibre ONT Lights: What They Mean
Use the exact Power, PON, LOS and LAN pattern to identify whether the failed layer is optical, provisioning, Ethernet, router or power.
Before touching the equipment
Photograph the ONT Lights Before Rebooting Anything
The original light combination is the most useful evidence. Do not look into, unplug or repeatedly bend the optical connector.
Light → layer → action
Match the ONT Light Pattern
Optical signal unavailable
Check only for an obvious kink or pinch, photograph the lights and contact the retail provider. Router settings cannot repair this layer.
Registration or provisioning
The fibre signal may be present while the ONT is waiting for activation or registration. Report the exact model and pattern.
Ethernet handoff
Use a known-good cable between ONT LAN and router WAN and inspect the router port and negotiated link.
Router or provider session
The optical layer is probably healthy. Check router WAN authentication, DHCP, VLAN or provider service status.
What Does a Red or Flashing LOS Light Mean on a UK Full-Fibre ONT?
A red or flashing LOS light means the ONT is not receiving a usable optical signal. Causes can include a damaged or sharply bent fibre lead, an external fibre fault, splitter or cabinet work, or severe attenuation. The exact source needs provider investigation.
Immediate action: photograph the lights, leave the optical connector in place and contact the retail broadband provider.
Why Is the PON Light Flashing While the LOS Light Is Off?
A flashing PON light while LOS remains off usually means the optical signal is present but registration or service activation has not completed. Possible causes include delayed activation, an ONT identity mismatch or another provisioning problem. The exact meaning still depends on the ONT model and provider.
Immediate action: check activation messages and report the exact PON pattern, installation date and ONT model.
Why Is PON Solid but the LAN Light Off?
A stable PON light with no LAN activity points away from the optical path and towards the Ethernet handoff. The ONT cable, router WAN port or link negotiation may be down even though the fibre service is established.
Immediate action: replace the Ethernet cable with a known-good cable and check the router WAN port.
Why Are All ONT Lights Normal but There Is No Internet?
Normal optical and Ethernet indicators mean the fault may sit in router authentication, DHCP, VLAN configuration or a provider session. Local Wi-Fi can continue broadcasting even when the router has no external internet address.
Immediate action: restart the router first and check service status. If some internet access remains, run the wired Broadband Speed Test; if nothing loads, use the broadband outage guide.
Lights stable but performance still fails? When PON and LAN remain normal but latency rises sharply under household load, run the Bufferbloat Test. When a wired connection stays established but requests intermittently fail, compare the Packet Loss Test before assuming an optical fault.
Fibre safety: keep the lead in a wide loop, do not look into the connector and do not repeatedly unplug it. Connector contamination or a fractured glass core needs provider repair.
Test in order
The Five-Step Full Fibre ONT Check
- Photograph Power, PON, LOS and LAN.
Capture the light pattern before restarting the router or ONT.Record: colour, solid or flashing, and timestamp.
- Inspect the fibre lead without unplugging it.
Look for a tight bend, pinch, pulled lead or visible damage and release pressure gently.Do not open or clean the optical connector yourself.
- Check ONT power.
Use a known-good wall socket and watch for flickering lights or repeated restarts.Unstable power: report the adapter or ONT behaviour.
- Check the ONT-to-router Ethernet cable.
Use a known-good cable, inspect the LAN light and confirm the router WAN port is active. If the LAN link stays up but traffic intermittently fails, run the wired Packet Loss Test.PON stable but LAN changes: handoff fault.
- Escalate the failed layer.
Give the provider the exact lights, installation timing, local engineering context and Ethernet result. When all lights are healthy but service remains usable and slow, include a wired Broadband Speed Test result.Persistent LOS or failed PON registration: provider action.
Advanced checks
Optical Registration, Fibre Bends and Ethernet Handoff
Macro-bend and micro-bend attenuation
A tight bend, pinch or damaged patch lead can allow light to escape from the fibre core. The outer jacket may look intact even when optical loss is high. Release pressure gently, but do not assume that straightening a fractured lead will restore service.
PON registration and provisioning
The ONT must register with provider equipment using its assigned identity and service profile. A provisioning mismatch can leave the optical signal present while service registration fails, which is why PON may flash with LOS off.
Ethernet negotiation between ONT and router
A damaged pair, loose clip or failed router WAN port can leave PON healthy but LAN dark or unstable. Use a known-good gigabit cable and check the router port before reporting an optical fault.
Do not assume a universal MAC lock
Some services retain DHCP or learned-client state, but ONTs do not universally bind permanently to the first connected device. Follow provider instructions before swapping equipment or performing extended power cycles.
Power-cycle only when appropriate
Restart the router first. Power-cycle the ONT only when provider instructions allow it. A controlled restart may clear a handoff state, but it cannot repair a broken fibre, failed optical path or rejected provisioning profile.
Escalate with evidence
What to Tell the Broadband Provider
ONT evidence
- Power, PON, LOS and LAN states
- Solid, flashing or completely off
- Exact ONT make or visible label
- Photo taken before rebooting
Fault context
- Installation or activation date
- Recent local engineering work
- Known-good Ethernet cable result
- Whether local router Wi-Fi still broadcasts
Useful fault report: “Wi-Fi is broadcasting, LOS is off, PON is flashing and a known-good Ethernet cable does not restore the router WAN connection.”
Common questions
Full Fibre ONT Light FAQs
What does a red LOS light mean on a full-fibre ONT?
A red or flashing LOS light normally means the ONT has lost the optical signal. Record the lights, check for an obviously kinked fibre lead and contact the retail broadband provider.
What does a flashing PON light mean when LOS is off?
The fibre signal may be present but the ONT may still be registering or waiting for activation or provisioning. Meanings vary by model, so report the exact pattern.
What does solid PON with no LAN light mean?
The optical path is probably established, but the Ethernet handoff to the router may be down. Check the cable, router WAN port and link negotiation.
Should I restart the ONT when full fibre is down?
Restart the router first. Power-cycle the ONT only when the provider permits it, and do not repeatedly restart equipment during an optical or activation fault.
Can I repair a red LOS fault by changing router settings?
No. Router settings cannot restore a missing optical signal. A persistent red LOS or optical alarm normally needs provider or network-operator investigation.