Quick summary: Your ONT is the small box full fibre plugs into. What every light means, what to do when one changes, and why you should never unplug the white cable.

The Fibre Box on Your Wall: What an ONT Is and What Its Lights Mean

If your internet is down right nowProvider guidance checked 10 August 2026

Healthy looks like this: POWER green, PON solid green, LOS off. If PON is off, or PON and LOS are flashing together, BT's guidance is that "Fibre optic link is not connected or there's a problem between your property and the BT exchange". Do this, in order: check "that the thin white fibre optic cable coming into your Openreach fibre modem is inserted and not damaged", then "Turn the Openreach fibre modem's power off, then back on and wait five minutes". If it does not come back, it is a line fault, so report it. Do not unplug, clean or re-route the thin white fibre cable, and do not unscrew the box.

lights that tell you almost everything: POWER, PON, LOS

5 min

wait after a power cycle before concluding anything

4 hours

how long Openreach says a full fibre install usually takes

18+

a decision maker must be present throughout the appointment

2 boxes

full fibre homes have an ONT and a router, not one device

official BT or EE guidance describing a red LOS light

When full fibre arrives, an engineer leaves a second box on your wall, and almost nobody explains it. It is called an optical network terminal, or ONT, and it is where the fibre from the street actually ends. This guide decodes every light in the providers' own words, gives you the exact sequence to follow when the internet drops, explains where the box can go and what you will be asked to sign on installation day, and answers the question that brings most people here: what a red light means. The honest answer to that last one is more interesting than you might expect.

The short version

The ONT is not your router. Openreach: "The ONT connects to your router and brings Full Fibre into your property." It has no Wi-Fi of its own.
PON is the light that matters. Solid green is healthy. Off, or flashing alongside LOS, means a problem beyond your walls.
The white cable is fibre, and it is delicate. Never unplug it, clean it or bend it sharply.
Position is decided on the day, near a double power socket and somewhere good for Wi-Fi, so think about it before the engineer arrives.
You will sign a permission form if drilling is needed, and renters need the landlord's agreement first.

What the lights mean, in the providers' own words

Direct answer: POWER green, PON solid green, LOS off is a healthy connection. Everything else is on this table.

BT and EE both publish guidance for the Openreach fibre modem, and the two are near identical because it is the same underlying equipment. Every meaning below is quoted from them.

Openreach fibre modem light states, quoted from BT's and EE's own help pages, checked 10 August 2026. Other networks use different equipment, so check your own provider's guidance if your box is not an Openreach one.
Light	State	What it means, and what to do
POWER	Green	"Openreach fibre modem is on."
POWER	Off	"Openreach fibre modem is off." Action: "Check the mains plug is in."
PON	Solid green	"Fibre optic link to BT exchange is working and recognises your Openreach fibre modem." This is the healthy state
PON	Flashing, LOS off	"Openreach fibre modem is communicating with BT exchange for verification." Action: "Wait until light turns solid green."
PON and LOS	PON off, or both flashing	"Fibre optic link is not connected or there's a problem between your property and the BT exchange." Action: check "that the thin white fibre optic cable coming into your Openreach fibre modem is inserted and not damaged", then "Turn the Openreach fibre modem's power off, then back on and wait five minutes"
PORT 1 to 4	On	"Ethernet connection is normal between Openreach fibre modem and Hub (Port 1 is where engineer will install)."
PORT 1 to 4	Blinking	"Data is being transmitted through the port." Normal in use
PORT 1 to 4	All off	"Ethernet cable to Hub is not inserted or broken." Action: "Check physical connection at both ends of the cable. Try new cable if needed."
TEL 1 to 2	On	"The fibre home phone line is live on your modem."
OPTICAL	Solid green	"Connected." BT notes this light on ECI models only

The red light question, answered honestly

Direct answer: neither BT nor EE documents a red LOS light at all, so treat any lit or flashing LOS the same way.

This is the single most searched thing about these boxes, and it deserves a straight answer rather than a confident guess. We read both major providers' official guidance for the Openreach modem on 10 August 2026. Both describe the LOS light in exactly two states: off, which is normal, and flashing alongside PON, which indicates a problem. Neither one mentions a red LOS state anywhere, despite red LOS being among the most commonly reported symptoms in UK broadband communities.

So what should you do if yours is red? Treat it as the documented fault state, because the meaning is the same: the optical link between your property and the exchange is not healthy. That means the fault is almost certainly outside your house, which is genuinely reassuring, because it also means there is nothing you can break by leaving it alone. Work through the sequence below, and if it does not clear, report it rather than experimenting.

When the internet drops: the sequence to follow

Direct answer: check the cable, power cycle, wait five minutes, then report it. In that order, and no further.

1 Read the lights before touching anything
POWER off means a mains problem, and BT's advice is simply to "Check the mains plug is in". PON solid green with LOS off means the line is fine and your problem is somewhere else, most likely the router or Wi-Fi.

2 Look at the thin white cable, do not touch it
Check it is "inserted and not damaged", in BT's words. Looking is the whole job. This cable carries light, its end face is precision made, and unplugging it to inspect or clean it is one of the few ways a customer can turn a temporary fault into an engineer visit.

3 Power cycle, then wait the full five minutes
"Turn the Openreach fibre modem's power off, then back on and wait five minutes for the Openreach fibre modem to attempt to reconnect." Five minutes is longer than most people's patience, and cutting it short is why plenty of working connections get reported as faults.

4 If the port lights are dark, suspect the Ethernet cable
All ports off means "Ethernet cable to Hub is not inserted or broken", and the fix is to "Check physical connection at both ends of the cable. Try new cable if needed." This is the one repair genuinely in your hands.

5 Still down? Report it, and start the compensation clock
A persistent PON or LOS problem is a line fault, not a home one. Report it to your provider, and note the date, because total loss of service can attract payments under the automatic compensation scheme. Our guide to automatic compensation rates covers what that is worth.

Where the box goes, and what you will be asked to sign

Direct answer: near a double socket and good for Wi-Fi, agreed with the engineer on the day, and yes there is a form.

The position is more negotiable than most people realise, and far less negotiable afterwards, so it is worth five minutes of thought before the appointment. Openreach's own installation checklist sets the constraints plainly. On placement: "The ONT connects to your router and brings Full Fibre into your property. Both should be positioned close together and near the double power sockets you prepared earlier." And on what makes a good spot: "It's got to be near a double power socket and a location best suited for WiFi coverage."

Because the ONT and router live together, this is really a decision about where your Wi-Fi will come from, which makes a hallway cupboard a worse idea than it first appears. The engineer will discuss "the best route for the fibre cable" and agree with you "where a small hole should be drilled", so arrive at that conversation with a preference rather than forming one on the spot.

On paperwork and permissions, three requirements from the same checklist are worth knowing in advance. "If drilling or digging is required, your engineer will ask you to sign a permission form before starting." Someone must be there with authority: "A decision maker over the age of 18 is there throughout the whole appointment." And for renters, this is the one to settle early: "If you rent your property, you have your landlord's permission for any drilling inside or outside."

On the drilling itself, Openreach is upfront: "We might need to drill a couple of holes outside just for our ladder safety. We will repair these holes and clean up after ourselves." And on time, "Your Full Fibre installation usually takes around four hours", which is the appointment to plan around. Our engineer visit checklist covers the rest of the day in detail.

Living with it: five things worth knowing

Direct answer: it is not yours, it is not your router, and it should be left alone.

It is network equipment, not your property. It stays with the address when you move, and you should not remove or relocate it. If you switch to another provider on the same network, the same ONT usually just carries your new service, which is part of why switching on full fibre is so simple.
It is not a router and never will be. No Wi-Fi, no settings, nothing to configure. When you want better Wi-Fi you change the router, and our guide to keeping your router when you switch covers that side.
Leave it powered. It draws very little, and switching it off takes down your connection and any phone service running over it, with a reconnection delay when you switch back. If your landline now runs over broadband, powering down the ONT means no calls, which matters most in the moments you would least want that.
The white cable is glass, not copper. Do not unplug it, do not coil it tightly, do not run furniture over it and do not attempt to clean the connector. Everything a household can usefully do is achievable without touching it.
A top tier upgrade may need a new one. Reporting on EE's 2026 multi-gigabit launch noted that customers "will also require an engineer visit to install one Openreach's latest 10Gbps capable Optical Network Terminals (ONT)". If you are eyeing the fastest packages, our guide to multi-gig broadband and the upload number is the place to start.

Beyond that, the ONT asks nothing of you. It is the quietest, least demanding piece of equipment in the house, and the main thing worth knowing is that a healthy one looks boring: two green lights and one that stays dark. If that is what you can see, your line is fine, and anything going wrong is happening somewhere you can actually influence. If you are still weighing up full fibre in the first place, our plain English guide to what FTTP is covers the basics, and an address check will tell you what is available at your door.

See what full fibre is available at your address

Free and independent. Address level, all networks, ranked by total contract cost.

Adrian's take

Writing this one, the thing that struck me was how much anxiety a small plastic box generates purely because nobody introduces it. An engineer spends four hours in your home, leaves two devices where there used to be one, and the second one has never been mentioned in any advert you have ever seen. Then one evening a light changes and you are searching at eleven at night with no internet, which is a spectacularly bad time to be reading a forum thread from 2019. What genuinely surprised me was the red light. It is the most asked question on the subject, and neither of the two biggest providers documents that state at all. I would rather tell you that than invent an answer, and the practical upshot is the reassuring part anyway: a PON or LOS problem lives outside your walls, so there is nothing in your house you can fix and nothing you can break by waiting. Read the lights, resist the urge to touch the white cable, give it five real minutes, then let somebody with a van deal with it.

Frequently asked questions

What is an ONT?

An optical network terminal, the small box a full fibre engineer fixes to an inside wall. The fibre from the street ends at the ONT, and the ONT connects by an Ethernet cable to your router, which is what actually creates your Wi-Fi. Openreach describes it simply: the ONT connects to your router and brings Full Fibre into your property. It is not a router and it has no Wi-Fi of its own, which is why a full fibre home has two boxes rather than one.

What do the lights on my ONT mean?

In normal operation the power light is green and the PON light is solid green, which BT describes as the fibre optic link to the exchange working and recognising your modem, with the LOS light off. A flashing PON with LOS off means the modem is communicating with the exchange for verification, and you should wait until the light turns solid green. PON off, or PON and LOS both flashing, means the fibre optic link is not connected or there is a problem between your property and the exchange.

What does a red light on my ONT mean?

Officially, less than people assume. BT and EE both publish guidance for the Openreach modem, and both describe the LOS light only as off or flashing; neither documents a red state, even though red LOS is widely reported by customers. The safe reading is to treat any lit or flashing LOS the same way, as a signal problem between your property and the exchange. Check the thin white fibre cable is properly inserted and undamaged, power cycle the unit, wait five minutes, and if nothing changes report a fault rather than trying to fix it yourself.

What does LOS mean on a fibre modem?

Loss of signal. It is the light that tells you the optical connection between your home and the exchange is not healthy. In normal use it should be off. BT's guidance pairs it with the PON light: PON solid green with LOS off is working, while PON off or PON and LOS flashing together indicates the fibre optic link is not connected or there is a problem between your property and the exchange.

My internet is down and the PON light is off. What do I do?

Follow the providers' own sequence. Check that the thin white fibre optic cable coming into the modem is inserted and not damaged. Turn the modem's power off, then back on, and wait five minutes for it to attempt to reconnect. If the PON light does not return to solid green after that, the problem is on the line rather than in your house, so report a fault to your provider. Do not remove or clean the fibre cable and do not unscrew the unit.

Can I choose where the ONT goes?

Within limits, and the conversation happens on the day. Openreach's checklist says the ONT and router should be positioned close together and near the double power sockets you prepared earlier, and that the spot has got to be near a double power socket and a location best suited for WiFi coverage. The engineer will discuss the best route for the fibre cable and agree with you where a small hole should be drilled. So the practical answer is that you have real input, bounded by where power is and where the cable can sensibly enter.

Can I move my ONT after installation?

Not yourself. The fibre is terminated at the box and the cable is not something to pull, bend sharply or re-route, and the unit is fixed to the wall as part of the installation. If the position genuinely does not work, speak to your provider about whether a repositioning visit is possible rather than attempting it. This is the strongest argument for thinking about the location before the engineer starts rather than after.

Do I have to sign anything on installation day?

If drilling or digging is involved, yes. Openreach's checklist states that if drilling or digging is required, your engineer will ask you to sign a permission form before starting. It also asks that a decision maker over the age of 18 is there throughout the whole appointment, and that if you rent your property, you have your landlord's permission for any drilling inside or outside. Renters should settle that permission before the appointment, not on the doorstep.

Will the engineer drill holes in my walls?

Usually at least one, for the fibre to enter, and the position is agreed with you on the day. Openreach's checklist says the engineer will agree where a small hole should be drilled, and separately that we might need to drill a couple of holes outside just for our ladder safety, adding that we will repair these holes and clean up after ourselves. Knowing this in advance is the difference between a surprise and a decision.

How long does a full fibre installation take?

Openreach's own guidance is that your Full Fibre installation usually takes around four hours. That is the appointment to plan for, and someone over 18 needs to be present throughout. It is worth having decided in advance roughly where you want the equipment, because that conversation happens early and the rest of the visit follows from it.

What is the difference between an ONT and a router?

The ONT converts the light signal arriving on the fibre into something your equipment can use, and it does nothing else. The router takes that connection and shares it around your home, over cable and Wi-Fi, and handles all the network features. You can change your router when you switch provider or upgrade your Wi-Fi. The ONT belongs to the network and stays where it is.

Can I unplug my ONT to save electricity?

It is not worth doing. The unit draws very little, powering it down takes your connection offline entirely including any phone service that runs over it, and reconnection is not instant because the link has to be re-established with the exchange. If a household phone service depends on the connection, losing power also means losing the ability to make calls, which matters most in exactly the situations where you would want a phone.

Do the port lights on the ONT matter?

They tell you whether your router is talking to it. BT describes a lit port as meaning the Ethernet connection is normal between the modem and hub, noting that Port 1 is where the engineer will install, and blinking as data being transmitted through the port. If all the port lights are off, the guidance is that the Ethernet cable to the hub is not inserted or broken, so check both ends of that cable and try another if you have one.

Do I need a new ONT for faster broadband?

For ordinary upgrades, no, the same unit carries a range of speeds. For the newest multi-gigabit tiers it can be a different story: reporting on EE's 2026 multi-gig launch noted that customers would require an engineer visit to install one of Openreach's latest 10Gbps capable optical network terminals. So a jump to a top tier may mean an appointment rather than a remote change, which is worth factoring into your timing.

Who owns the ONT, me or the provider?

Not you. The ONT is network equipment installed at your property, which is why you cannot take it with you when you move and should not attempt to remove or relocate it. If you switch to another provider using the same network, the same ONT normally stays and simply serves your new provider's service, which is one reason switching on full fibre is usually straightforward.

References

BT. (n.d.). What do the lights on the Openreach modem mean? https://www.bt.com/help/landline/lights-on-the-openreach-modem
EE. (n.d.). What the Openreach modem lights mean. https://ee.co.uk/help/broadband/fix-problem/what-the-openreach-modem-lights-mean
Openreach. (n.d.). What to expect during your Full Fibre installation. https://www.openreach.com/help-and-support/full-fibre-broadband-installation-checklist
Jackson, M. (2026, July 31). ISP EE UK launch 2.3Gbps and 8Gbps broadband and confirm prices. ISPreview UK. https://www.ispreview.co.uk/index.php/2026/07/isp-ee-uk-launch-2-3gbps-and-8gbps-broadband-and-confirm-prices.html
Ofcom. (2026, April 1). Automatic compensation for broadband and landline customers. https://www.ofcom.org.uk/phones-and-broadband/service-quality/automatic-compensation-need-know

Written by Adrian James, Broadband Editor at BroadbandSwitch.uk (LinkedIn). Reviewed by Dr Alex J. Martin-Smith (LinkedIn). Published 10 August 2026, provider guidance checked 10 August 2026. Light meanings quoted here are for the Openreach fibre modem as documented by BT and EE; other networks use different equipment, so check your own provider's guidance if your box is not an Openreach one. This guide is information, not advice.