T3 and T4 Timeout errors - frequent connectivity outages

I have been dealing with constant internet outages for going on 3 and a half months. I've gotten a "new" replacement MB8611 modem from Motorola and an Xfinity tech came out to run a new line from the pedestal to the house. Still getting 10 or more outages per day.

The closest I can come to a diagnosis is what the Motorola engineer who is working on this has shared with me based on his read of the modem Connection and Event Logs pages. His email:

Your coaxial network connection is pretty bad. According to the Status Connection page you provided, there are a huge amount of Uncorrected errors indicating a big packet loss on the coaxial line network provisioned by your ISP.
The power levels(Pwr) for the downstream bonded channelsshould be between -7 and +7 dBmV(closer to zero the better it is) and within a 3dBmV range with all the downstream channels.
The upstream bonded channels should have power levelsbetween 38 - 50 dBmV, the optimum is 45 dBmV. Additionally, it's important that all upstream channels have power levels within a 3 dBmV range of each other.

In the Event Log I see many T3 timeout events. T3 timeout errors mean that the cable modem has sent 16 Ranging Request (RNG-REQ) messages without receiving a Ranging Response (RNG-RSP) message in reply from the CMTS.
The cable modem is therefore resetting its cable interface and restarting the registration process. This typically is caused by noise on the upstream that causes the loss of MAC-layer messages.
T4 timeout events which are also present in your event are a more severe error than T3, meaning the modem lost the connection because of a lack of response from the ISP.

I recommend contacting your Internet Service Provider (ISP) and requesting a technician visit. Ensure that they rectify and optimize all the values to fall within the specified optimal parameters.

Here are the Connection and Event Logs I have gathered from the modem and shared with him (stripped the CM-MAC and CMTS-MAC info out). Earlier in the day on May 7 were multiple T4 errors (I can provide to a tech, but I only have the screenshots and those include the CM-MAC and CMTS-MAC info):

Event Log

Time	Priority	Description
01:07:29 Wed May 7 2025	Critical (3)	No Maintenance Broadcasts for Ranging opportunities received - T2 time-out
01:07:34 Wed May 7 2025	Critical (3)	UCD invalid or channel unusable
01:07:53 Wed May 7 2025	Critical (3)	UCD invalid or channel unusable
01:07:54 Wed May 7 2025	Critical (3)	No Ranging Response received - T3 time-out
01:07:56 Wed May 7 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
07:45:33 Wed May 7 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
15:15:41 Wed May 7 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
08:30:39 Thu May 8 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
10:15:39 Thu May 8 2025	Critical (3)	UCD invalid or channel unusable
10:15:44 Thu May 8 2025	Critical (3)	Started Unicast Maintenance Ranging - No Response received - T3 time-out
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
13:59:26 Thu May 8 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
14:01:23 Thu May 8 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
14:04:51 Thu May 8 2025	Critical (3)	UCD invalid or channel unusable
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable
Time Not Established	Notice (6)	Honoring MDD; IP provisioning mode = IPv6
15:15:32 Thu May 8 2025	Notice (6)	DS profile assignment change. DS Chan ID: 32; Previous Profile: ; New Profile: 1 2 3.
15:15:42 Thu May 8 2025	Critical (3)	UCD invalid or channel unusable

Downstream Bonded Channels

Channel	Lock Status	Modulation	Channel ID	Freq. (MHz)	Pwr (dBmV)	SNR (dB)	Corrected	Uncorrected
1	Locked	QAM256	4	405.0	5.1	43.7	0	0
2	Locked	QAM256	1	387.0	4.8	43.8	0	0
3	Locked	QAM256	2	393.0	4.9	43.7	0	0
4	Locked	QAM256	3	399.0	5.0	43.6	0	0
5	Locked	QAM256	5	411.0	5.2	43.7	0	0
6	Locked	QAM256	6	417.0	5.3	43.8	0	0
7	Locked	QAM256	7	423.0	5.3	43.8	0	0
8	Locked	QAM256	8	429.0	5.4	43.8	0	0
9	Locked	QAM256	9	435.0	5.5	43.8	0	0
10	Locked	QAM256	10	441.0	5.5	43.8	0	0
11	Locked	QAM256	11	447.0	5.7	43.8	0	0
12	Locked	QAM256	12	453.0	5.8	43.8	0	0
13	Locked	QAM256	13	459.0	5.8	43.9	0	0
14	Locked	QAM256	14	465.0	5.8	43.9	0	0
15	Locked	QAM256	15	471.0	6.0	43.9	0	0
16	Locked	QAM256	16	477.0	6.0	43.9	0	0
17	Locked	QAM256	17	957.0	8.8	43.9	0	0
18	Locked	QAM256	18	963.0	8.8	44.0	0	0
19	Locked	QAM256	19	969.0	8.7	43.9	0	0
20	Locked	QAM256	20	975.0	8.5	43.9	0	0
21	Locked	QAM256	21	981.0	8.3	43.9	0	0
22	Locked	QAM256	22	987.0	8.0	43.8	0	0
23	Locked	QAM256	23	993.0	7.7	43.6	0	0
24	Locked	QAM256	24	999.0	7.5	43.2	0	0
25	Locked	OFDM PLC	193	774.0	9.5	44.2	1746915	0

Upstream Bonded Channels

Channel	Lock Status	Channel Type	Channel ID	Symb. Rate (Ksym/sec)	Freq. (MHz)	Pwr (dBmV)
1	Locked	SC-QAM	1	5120	16.4	36.3
2	Locked	SC-QAM	2	5120	22.8	36.5
3	Locked	SC-QAM	3	5120	29.2	36.5
4	Locked	SC-QAM	4	5120	35.6	36.5

What more can I provide? I really want to email someone senior at Xfinity to share the log info, if that would help but there doesn't seem to be any sort of option that I've seen that would allow me to do so... yet.

I feel like I have hit a wall in the standard tech support escalation and need senior tech support or I'm just going to keep going in circles.

Responses

Yesgaro

Frequent Visitor

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10 Messages

15 days ago

Just an update to include more additional T3 and T4 errors in the logs from this morning... had multiple outages in the middle of work calls where I was screen sharing which was unpleasant.

02:41:23 Fri May 9 2025	Critical (3)	Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out;
02:41:50 Fri May 9 2025	Critical (3)	UCD invalid or channel unusable;
02:41:55 Fri May 9 2025	Critical (3)	No Ranging Response received - T3 time-out;
02:42:21 Fri May 9 2025	Critical (3)	UCD invalid or channel unusable;
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable;
08:15:42 Fri May 9 2025	Critical (3)	UCD invalid or channel unusable;
Time Not Established	Critical (3)	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;
Time Not Established	Critical (3)	UCD invalid or channel unusable;
Time Not Established	Notice (6)	Honoring MDD; IP provisioning mode = IPv6
08:18:39 Fri May 9 2025	Notice (6)	DS profile assignment change. DS Chan ID: 32; Previous Profile: ; New Profile: 1 2 3.;
08:18:48 Fri May 9 2025	Critical (3)	UCD invalid or channel unusable;
08:24:02 Fri May 9 2025	Notice (6)	CM-STATUS message sent. Event Type Code: 24; Chan ID: 193; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 0.;

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XfinityQuemekia

Official Employee

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133 Messages

Hi @Yesgaro, Thank you for reaching out to us here on our Xfinity Community Forums. We are sorry to hear about the problems you're having with your internet connection. Have you tried any additional troubleshooting? Are you losing connection on all of your devices or specific devices?

Here's a great checklist for troubleshooting by one of our experts: https://forums.xfinity.com/conversations/your-home-network/internet-troubleshooting-tips/602dae4ac5375f08cde52ea0.

I am an Official Xfinity Employee.
Official Employees are from multiple teams within Xfinity: CARE, Product, Leadership.
We ask that you post publicly so people with similar questions may benefit from the conversation.
Was your question answered? Please, mark a reply as the Accepted Answer. tick

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13 days ago

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Yesgaro

Frequent Visitor

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10 Messages

Hi there, thanks for the response... I know you need to check, but I've had over three months of troubleshooting, so I've had ample opportunity to:

Check for outages
Verify that the MB8611 is still an approved modem (it is)
Get a new MB8611 from Motorola (they did)
Determine if the issue is post-modem wired and/or wifi (it isn't, see below)
Reset my equipment... I have performed soft and factory resets of both the modem and wifi routers so many times, I couldn't tell you. But it has to be approaching 100 or more at this point.
I have a dedicated motorola engineer performing support on this issue and his read on the modem connection and event logs is as I quoted in the original posting, that it is excessively noisy on the COAX side and that is contributing to the issue. The additional/related concern he shared was the T3 and T4 timeouts that would be entirely on the Xfinity side. Because Xfinity hasn't provided any more senior tech attention to the issue aside from dispatching techs to run a new line, I have to take his word for it.

With respect to trying different hardware and network configs, I have replaced the modem completely, and have tried the router after it with a wifi router, a laptop running as a DHCP server, and a DHCP server running on a connected NAS (Diskstation running a Linux OS). Regardless, when internet goes out, it goes out for all wired and wifi devices.

Let me know if there is any more info I might provide to help escalate this...

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13 days ago

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XfinityMatthew

Official Employee

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189 Messages

All of that is great. Appreciate you being thorough and doing your due dilligence to rule out possible problems. For further troubleshooting we will want to get into the account and take a look at everything. To get started. Please send us a direct message with your full name and the address.

To send a "Direct Message" to Xfinity Support:
Click "Sign In" if necessary
Click the "Direct Messaging" icon or https://forums.xfinity.com/direct-messaging
Click the "New message" (pencil and paper) icon
The "To:" line prompts you to "Type the name of a person". Instead, type "Xfinity Support" there
- As you are typing a drop-down list appears. Select "Xfinity Support" from that list
- An "Xfinity Support" graphic replaces the "To:" line
Type your message in the text area near the bottom of the window
Press Enter to send it.

I am an Official Xfinity Employee.
Official Employees are from multiple teams within Xfinity: CARE, Product, Leadership.
We ask that you post publicly so people with similar questions may benefit from the conversation.
Was your question answered? Please, mark a reply as the Accepted Answer. tick

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13 days ago

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EG

Expert

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110.5K Messages

12 days ago

@Yesgaro @XfinityMatthew

Please circle back here and post any possible solutions for the issue here in these open public forums so that all readers here may benefit from the exchange / info. This is in keeping with the spirit for which these public help forums were originally intended. Thank you.

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Yesgaro

Frequent Visitor

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10 Messages

8 days ago

Alrighty, I had a senior Xfinity tech come out to the house last weekend and have been monitoring my connection this week. I shared the Motorola tech's feedback on line noise and T3 and T4 outages with him from the connection and event logs. Some notes from that visit:

He ran tests on the line regarding noise and said it appears perfectly clear on the Xfinity side.
He also tested the line up to the modem, without the modem connected and the noise was within specs according to him. When he reconnected the modem, he said the noise spiked. On my request he supplied screenshots of the before and after which are attached (IMG_0953 is without the modem, IMG_0954 is with it).
He mentioned that the local node had been recently upgraded to fiber, if that is of any relevance
I showed him the T3 and T4 timeouts in the logs and he looked up the times and said he didn’t see any corresponding outages. Additionally, he said that if there had been T3 and T4 timeouts it would have affected the whole neighborhood.
On a similar note, while he was here, with the Motorola hooked up, we saw two more T4 timeouts in the space of an hour, reported in the Motorola logs. So he was able to immediately check while here, but didn’t see anything on the Xfinity side where there were associated service issues, and the neighbors that are all on the same node didn’t see outages that he said would have also resulted from that sort of critical error.
They agreed to give me one of their modems for 30 days free of charge, to test. After a week with the Xfinity modem the internet has not gone down once, except for a software update to the Xfinity modem at 4am Monday morning.

I am attaching screenshots from the tech that I asked he take and share while he was testing so I would have something to share back with you as most of it is beyond my technical ken. In so many words, he said that the errors that the modem was reporting were false. That there were no out of spec noise or timeouts that correspond to what the MB8611 is reporting. Additionally, as mentioned above, the only noise spike that he said he saw was once the Motorola was re-attached to the line and the whole range jumped (the before and after heat map screenshots).

When I went back to the Motorola support with all of this, his response was basically that I should still be demanding Xfinity meet Motorola's requirements for line noise management:

To address the coaxial connection parameters that should have been fixed by the Xfinity technician during their visit, these specifications are standard across most cable modems. However, rental modems provided by ISPs are built specifically for their network infrastructure. They allow real-time access and continuous firmware updates customized to meet the ISP’s performance and compatibility requirements, which can make them more tolerant of signal inconsistencies that may affect retail modems.

To address the coaxial connection parameters, please install the following coaxial attenuator right before the modem at the end of the coaxial cable: https://www.amazon.com/6db-Attenuator-Pad-1-each/dp/B0013L48XA

So, what may be the community's suggestion of how to proceed? I clearly am not in a position to be arbitrating a technical dispute between two multi-billion dollar companies, despite being out $200 for the modem. And another $6 for the attenuator.

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Your Home Network

Yesgaro

T3 and T4 Timeout errors - frequent connectivity outages

Yesgaro

XfinityQuemekia

Yesgaro

XfinityMatthew

EG

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