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Openreach fibre modem (ONT): what the lights mean

If you have full fibre broadband, there may be a small white box on the wall called an Openreach modem or ONT. A thin fibre cable goes into it, and an Ethernet cable runs from it to your router. This guide follows BT's help page on its lights. Models vary, so check the label on your box.

What the lights mean

  • Power (right): green means on. Off means no power, so check the plug.
  • PON (left), solid green: the fibre link to the exchange is working and the box is recognised.
  • PON flashing: the box is checking in. Wait for it to go solid green.
  • PON off, or PON and LOS both flashing: there is a connection problem. Check the thin white fibre cable is fully inserted and not damaged, then switch the box off and on.
  • Port lights 1 to 4: on is a normal Ethernet connection, blinking means data is moving, and all off means the Ethernet cable to your router is missing or faulty.
  • Tel lights: only relevant if you have a phone service. Blinking means the line is not activated yet.

What to do

  1. Check the box has power. Do not move it or unplug it from the wall socket without a good reason.
  2. Check the Ethernet cable runs from a port on the box to the WAN or broadband socket on your router.
  3. Switch the box off at the plug, wait ten seconds, and switch it on. Wait several minutes.
  4. If PON is not solid green, do not bend or pull the fibre cable. It can break or lose signal.

Please be careful

The fibre cable is delicate. Never look into the end of a fibre cable or force it. If it is damaged, leave it for an engineer.

When to get more help

Contact us or your broadband provider if PON or LOS lights stay wrong after a restart, if the fibre cable is damaged, or if the box has no power even though the socket works. Your provider reports the fault to Openreach, so tell them the lights you see. Do not try to repair the box yourself.

Did this help? If you are still stuck, email Vision Tech and tell us what you can see on the screen. Menus and button names can differ slightly between versions and makes. If something does not match, stop and ask rather than guess.

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