What should I check first for full fibre ont fault?

Start by validating the scope of the issue: check whether it affects one device, one room or the whole connection, then compare Wi‑Fi and Ethernet where possible.

Can Wi‑Fi cause full fibre ont fault?

Yes. Weak Wi‑Fi, interference, router placement and device limitations can cause many broadband symptoms even when the line itself is working.

When should I contact my provider?

Contact your provider if the problem affects Ethernet as well as Wi‑Fi, router or ONT lights show a service fault, or repeated tests prove the issue is outside your home network.

Full Fibre ONT Lights Meaning | Validate LOS, PON and Router Faults

Full fibre validation guide

Full Fibre ONT Lights Meaning

This guide helps you validate full fibre faults by checking ONT lights such as power, PON, LOS, optical, LAN and service indicators before contacting your provider.

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Full fibre ONT lights troubleshooting illustration showing LOS, PON and internet status indicators

Issue

Symptoms of Full Fibre ONT Fault

Use these signs to confirm that this is the closest matching issue before changing settings, replacing equipment or contacting your provider.

Full fibre broadband is down or unstable.
The ONT shows LOS, optical, PON or alarm light changes.
Router Wi‑Fi works locally but there is no internet service.
The router WAN/Internet light is off or cannot connect through the ONT.
Service drops after fibre installation or during provider faults.

Likely causes

Most Common Causes

The same symptom can have several different causes. Start with the causes below, then use the validation steps to prove which one is most likely.

Loss of optical signal

LOS or optical warnings can indicate the fibre signal is missing or impaired.

ONT power issue

No power or unstable power prevents the ONT from serving the router.

Router-to-ONT cable issue

A loose or faulty Ethernet cable between ONT and router can look like no internet.

Provider provisioning fault

A service may not be activated or authenticated correctly.

Area network issue

Full fibre faults can happen outside the property and need provider repair.

Validate

Steps to Narrow Down the Root Cause of the Issue

Work through these checks in order. Change one thing at a time so the result tells you something useful.

1
Check ONT power first.
2
Look for LOS/optical/PON/service light status and write it down.
3
Check the Ethernet cable from ONT to router WAN port.
4
Restart router only first, then ONT only if your provider instructions allow it.
5
Test whether devices can connect to router Wi‑Fi even when the internet is down.
6
Check provider status and installation/activation messages.

Fix

Problem Resolution

Apply the fix that matches the cause you validated. If the issue is proven outside your home network, gather evidence before contacting your provider.

Do not bend or disturb fibre leads

Fibre cables are fragile; avoid unplugging optical leads unless instructed.

Reseat Ethernet and power safely

Check ONT power and router WAN Ethernet before assuming a provider fault.

Use light evidence when contacting support

Tell your provider the exact ONT light state, router state and whether Wi‑Fi is still broadcasting.

Request provider repair for optical faults

LOS/optical faults usually require provider or network operator action.

Next guides

Related broadband troubleshooting guides

If this page does not exactly match the issue, use one of these related guides from the broadband issue hub.