This topic

Flashing PON light on ONT

bdyer — Sun, 05 Oct 2025 11:32:40 GMT

Hi all,

I had fibre to the home installed last December. Everything has been fine until a couple of weeks ago when I started to get intermittent connection issues around similar times of day where connection would be lost for 5-30m each time. This tended to occur once a day, but some days would be fine. There are some examples in the table below. Each time the connection went down, the PON light flashed on the ONT.

Equipment: ONT, connected to BT Home Hub, EE backup hub connected via ethernet (frankly not great at its job), Unifi system connected via ethernet, The Unifi equipment serves the rest of the house with WiFi/wire connections/cameras etc.

I've had a BT engineer visit who did some tests, couldn't find anything, then escalated to an Openreach engineer. The engineer ran many tests over the course of an hour or so, replaced the ON. The Openreach engineer seemed stumped and said that according to the Openreach team, there were no issues detected with my "build" (presumably my configuration). The Openreach engineer even saw it PON flashing happen in real time when he was with me. The same outage happened again today.

Could there be a scheduled job/process running in the exchange that's causing this outage? The issue appears too regular to be related to a local fault.

Has anyone else had a similar issue or can offer any guidance/suggestions please? I will contact BT again and report the problem hasn't been fixed. Also I have download edthe BT hub logs, so if anyone knows any clues I can look for in those, please let me know!

Thanks,

Ben.

Example timings of outages (trimmed for brevity)

Restored	Today at 11:15:04
Down	Today at 11:09:02
Restored	Oct 2, 11:41:06
Down	Oct 2, 11:29:30
Restored	Sep 29, 11:04:53
Down	Sep 29, 11:04:29
Restored	Sep 28, 11:30:43
Down	Sep 28, 10:59:05
Restored	Sep 27, 13:17:13
Down	Sep 27, 13:09:02
Restored	Sep 25, 10:53:41
Down	Sep 25, 10:53:38

Re: Flashing PON light on ONT

iniltous — Sun, 05 Oct 2025 12:48:28 GMT

A flashing PON light is the ONT and OLT ( headend ) ‘handshaking’ basically its the authentication protocol of the ONT being allowed onto the ‘network’ , in layman’s terms , the ONT is asking to be connected, the headend is replying ‘ hang on while I check you are a valid ONT ‘ once the authentication is done , the PON light goes steady green , if the OLT he no ‘record’ of the ONT serial number , authentication would fail and the PON light would never go steady green but continue to flash …it’s not likely that there is a situation where the headend data were somehow corrupt and ‘forgetting’ only your serial number, causing a new authentication cycle just for you while leaving all other members on the PON working without issue .

As you seem to have had the ONT replaced (that’s what I would have suspected as the problem ) that effectively removes the ONT from the list of possible faulty components , however your service is optically identical to all the other users on your ‘PON’ its difficult to see how a headend /OLT problem would only affect you and not all the other users (potentially 30 users in total) , you could be the only customer on the PON , or the only one to have reported it of course.

Was the power unit replaced at the same time as the ONT ? , it’s possible (but not really that likely) if the power drops , even if it’s only momentarily, the ONT and OLT will go through the authentication process each time , although the varying times to re authenticate your connection don’t seem to align with power ‘blips’ and you would also expect to see the power led go off and back on ….any issues in general with the power supply to the address ?

The authentication part of the network has nothing to do with your ISP , even if you stopped being a customer of anyone , the ONT would stay ‘connected’ to the headend , so whatever the issue is , it’s within Openreach’s domain , nothing to do with BT ….there is no ‘routine’ or scheduled maintenance that would cause this failure.

Do you know if any close by neighbours are on Openreach FTTP , connected to the same PON even if they were with a different ISP , optically you are identical , the same aggregated data arrives at all ONT’s , it’s the individual ONT that respond only the ‘ data packets’ addressed themselves, ignoring everyone else’s data …so if it were the headend ‘falling over ‘ , all users would see the outage not just you , perhaps ask if anyone really close by is experiencing the same issues .

Re: Flashing PON light on ONT

bdyer — Sun, 05 Oct 2025 12:45:27 GMT

Thanks for the quick response - really useful infor and perspective - thak ou. I don't believe the ONT PSU was replaced. I'm still left wondering why this is happening at a reasonably regular time nearly every day. As you suggest, it's likely that it's affecting other people and I would expect Openreaches' own systems would be able to see this issue, but apparently they cannot.

Completely random thought: could a local brownout of the electricity supply cause this disconnection/reconnection. I wonder if there's a supply/demand issue on the local electricity supply?

Re: Flashing PON light on ONT

tonysmini018 — Sun, 05 Oct 2025 20:28:47 GMT

It could be a rogue ONT in a neighbouring house/business on your PON (up to 30 properties will share a single fibre from the OLT head end).

I’ve heard of a couple of faults where things like this happen, it may be worth asking BT to get Openreach to check if there are any other ONT’s alarming as well as yours.

Re: Flashing PON light on ONT

36ULW — Mon, 06 Oct 2025 18:14:28 GMT

32, Openreach use 32 Way SASA’s.

Re: Flashing PON light on ONT

Edinburgh-wg — Tue, 07 Oct 2025 05:51:45 GMT

If it's not PSU, what about your socket?

What about this: https://community.bt.com/t5/BT-Fibre-broadband/Flashing-PON-light/m-p/2356962#M352738

Re: Flashing PON light on ONT

bdyer — Tue, 07 Oct 2025 07:21:57 GMT

Thanks, @Edinburgh-wg

To try to rule out some variables, I've just added a UPS - https://cpc.farnell.com/apc/be850g2-uk/ups-back-ups-520w-850va/dp/CS33829 I have the ONT, BT Hub and EE backup hub plugged into it (plus another for my Unifi gear which is in a different part of the house). This should protect from local brownouts and general power loss. It could still be the ONT PSU, but the given the regularity of the failures (mostly just before and after 11am), it seems unlikely.

Thank you for the link, that's definitely an angle worth exploring if I continue to see this issues beyond today.

Thanks everyone for the tips and guidance, I'll keep you posted!

Re: Flashing PON light on ONT

bdyer — Sun, 12 Oct 2025 19:36:18 GMT

Update: unfortunately the intermittent connection has happened multiple times since the OR engineer replaced the ONT on Oct 4th. I've ruled out brownouts as the issue has happened multiple times since I added the UPS.

I was expecting an OR engineer tomorrow, but just checking MyBT site, I can see that the appointment has been cancelled because "Your speed issue has been fixed" (with no notification to let me know).

Is there any way via this forum that I can get in touch with technical people who can help escalate this internally please? This is clearly an unusual issue that needs a more nuanced and technical approach than the standard customer service approach can provide. It's frustrating.

Thanks

Re: Flashing PON light on ONT

Lesley_W — Mon, 13 Oct 2025 13:08:39 GMT

Hi @bdyer

Welcome to our community.

I'm sorry you are still having trouble with this and your appointment has been cancelled.

We don't have account access here on the forum. Have you logged a complaint about this yet? If not you can do so here Make a complaint.

Lesley

Re: Flashing PON light on ONT

bdyer — Mon, 13 Oct 2025 17:02:29 GMT

Thanks @Lesley_W

I spoke with two of the BT support team ealier who were extremely helpful and raised a complaint on my behalf. Having now canvassed my street via the WhatsApp group (which I remember an earlier poster had suggested) it seems that 10 or more people are having intermittent issues connection issues and variables speeds. This will hopefully point Openreach in a direction where they can investigate further.

Re: Flashing PON light on ONT

Lesley_W — Mon, 13 Oct 2025 17:37:19 GMT

Thanks for the update @bdyer

I'm pleased the team are looking into it. While it's never great for others to be having issues too, it does, as you say, help to know this when trying to narrow down the cause.

Hopefully they find the cause soon and you are back to normal.

Lesley

Re: Flashing PON light on ONT

iniltous — Tue, 14 Oct 2025 09:48:22 GMT

If near neighbours , so likely to be on the same PON as the OP , are having the same problem , then that changes everything, from looking at the issue as one entirely within the OP’s ‘domain’ , the ONT , the PSU , the house power supply, to suspecting the PON itself, from the headend (OLT) to the splitter node ….as the OP indicates the PON light flashes rather than the LOS (loss of signal) being illuminated suggests a headend issue rather than the physical medium (the optical cable ) , OLT issues really can’t be the problem if every other user on the PON wasn’t also experiencing issues , hence the necessity to establish if multiple users are affected or just one.

I recall a problem posted on here a while ago , not mentioned previously as the indication was only the OP was experiencing issues , there was a problem with the SFP for the affected PON at the headend , the OLT trained Openreach tech told the customer ( and poster on here ) there were a faulty batch of SFP’s fitted at multiple OLT’s , and a program to identify and rectify the issue was being rolled out , it’s possible that the OP is on a PON with an SFP from this batch of potential unreliable SFP’s , if the OP manages to speak to someone ‘au fiat’ with that issue affecting some head ends , (rather than a tech that deals exclusively with the external network who may not necessarily be aware of that problem ) it may be something worth investigating .

Re: Flashing PON light on ONT

pddco — Tue, 14 Oct 2025 20:18:14 GMT

@36ULW SASAs? I’m sure there’s others like me who appreciate the technical detail in these posts. But sometimes the TLAs throw me..

@iniltous Oh, and there goes another one.. SFP?

Thought I was doing alright but just occasionally they trip me up. Thanks.

Re: Flashing PON light on ONT

licquorice — Tue, 14 Oct 2025 20:30:27 GMT

@pddcowrote:

@iniltous Oh, and there goes another one.. SFP?
Thought I was doing alright but just occasionally they trip me up. Thanks.

Small Form Factor Pluggable.

It allows various types of interface to be plugged into a chassis rather than having dedicated sockets. They can be optical interfaces or Ethernet of various speeds and flavours.

Re: Flashing PON light on ONT

bdyer — Tue, 14 Oct 2025 20:32:08 GMT

@iniltous Thank you for the great insight and tech details. I'm supposed to be getting a visit from an Openreach SFI tomorrow. The appointment is booked, but no SMS to confirm. If they're able to come, I'll share this info with them.

Thanks!

Re: Flashing PON light on ONT

iniltous — Wed, 15 Oct 2025 09:07:46 GMT

FYI ….SASA , splitter array sub assembly, basically the equipment that takes a single input ‘fibre’ and splits it into 32 output fibres ( that’s the basis of passive optical networks, it’s a passive split , no power needed , it’s an optical rather than electronic device ) , and as stated , SFP (small form plugable transceiver ) a device that transmits and receives , it converts optical signals into electrical signals and electrical signals in to optical ones , there is a also transceiver in the ONT but it’s not a SFP as it’s not ‘plugable’ (a separate device) …its a component built onto/into the ONT

Re: Flashing PON light on ONT

bdyer — Fri, 24 Oct 2025 18:48:04 GMT

Hi all, a brief update on this and what I hope is a happy ending!

A 4th engineer came on the 17th Oct and ran all the normal tests, found nothing. No issues with the build. The engineer then opened up the two manhole covers on the street and rerouted my connection into what I think he said was a spare/unused cable. Since then, I've had no dropouts.

I'd love to hear the view from the group how this could have fixed the issue and how it could help others in the future. Of course, this could be a coincidence and the problem was fixed elsewhere on the same day.

Thanks again for your help.

Re: Flashing PON light on ONT

iniltous — Sat, 25 Oct 2025 08:15:02 GMT

In very general terms , anyone with FTTP availability, will only have access to a single PON (it gets complicated if an address has access to multiple PONs (passive optical networks ) do you know if the tech had to re authenticate your ONT onto a different PON ?.

To maximise efficiency, (if the local geography allows it ) a node will be designed to have 4 splitters within it ,( think of each splitter as a PON , with its own unique ID ) and each splitter designed to have 30 addresses , so to simplify, say you were on a road with 60 houses , numbers 1 to 30 designed to be on splitter A , via multiple CBT’s , and houses 31 to 60 on splitter B via multiple CBT’s , it’s possible that (for example ) you could use a spare port on a CBT from splitter B for a house numbered 1-30 , but it would mess up the design, potentially leave a an address in the 31 to 60 range short of a port (although there are 32 ports , 30 is a self imposed limit ) , and the duct network would need to have access to both , and the address would have a longer cable route to the CBT than originally designed, but it could be possible , just not desirable ….if the tech did this , they would have to re authenticate your PON from one splitter ID to the other splitter ID .

If ( and this is more likely ) they simply connected you to spare port on the same PON , nothing actually changes optically as ports are identical if on the same splitter , what would change is the ‘mechanical’ connection , the devices in the joint boxes (and up poles for that matter ) are CBT’s , connectorised block terminals , a plug and socket arrangement, if the plug and socket were defective in a way that occasionally the connection failed (that’s possible) it would be weird that the issue presented itself in the way it did ( regularly at around the same time of day ) .

Hopefully this is the fix and it’s not a problem that will return, but the fix (unless the tech did move you to a different splitter ) seems peculiar, and obviously if it were an issue with the PON ( the one you were on ) and that issue wasn’t fixed , you were simply moved away from the faulty PON , which potentially will still be causing issues for those still connected to it ( you said some neighbours were also affected )

It’s possible the tech did something else , like changing the fibre from the aggregation node to the splitter , each splitter uses 1 fibre but has 3 allocated , perhaps it was that , although again it’s weird the way the problem manifested itself, dodgy splices or fibres generally work or don’t , they rarely work some of the time and not at other times , with a regular failure at around the same time of day , what’s more a light level check would usually indicate a poor connection that was barely ‘hanging on’ …but you can have remedies that work that seem to defy logic , hopefully whatever the engineer did fixed your problem and the neighbours problems at the same time .

Re: Flashing PON light on ONT

MichaelBrown864 — Sat, 25 Oct 2025 09:57:35 GMT

The flashing PON light usually means a signal loss or authentication issue between your ONT and the exchange. Since it happens at fixed times, it’s likely an upstream fault or scheduled maintenance at the exchange, not your home setup. Ask BT/Openreach to run continuous line monitoring during those hours to catch the signal drop in real time.