T3 errors with no resolution

FTD

I have had numerous techs out to my apartment with no solution in sight over the past 6 months. I keep getting a t3 timeout that basically puts my network in limp mode until it is rebooted. Techs have told me it is beyond their comprehension at this point as they have tried several solutions. I am using a CM3000 and have tried other 3.1 modems with mid-high split capability. On the 2.1 gb plan with 350 up.

Procedure	Status	Comment
Acquire Downstream Channel	645000000 Hz	Locked
Connectivity State	OK	Operational
Boot State	OK	Operational
Security	Enabled	BPI+
IP Provisioning Mode	Honor MDD	IPv6 only
 
Downstream Bonded Channels
Channel	Lock Status	Modulation	Channel ID	Frequency	Power	SNR	Correctables	Uncorrectables
1	Locked	QAM256	44	645000000 Hz	7.4 dBmV	44.3 dB	1	0
2	Locked	QAM256	13	459000000 Hz	7.7 dBmV	45.4 dB	0	0
3	Locked	QAM256	14	465000000 Hz	7.8 dBmV	45.4 dB	0	0
4	Locked	QAM256	15	471000000 Hz	7.8 dBmV	45.4 dB	0	0
5	Locked	QAM256	16	477000000 Hz	7.9 dBmV	45.4 dB	0	0
6	Locked	QAM256	17	483000000 Hz	7.9 dBmV	45.4 dB	1	0
7	Locked	QAM256	18	489000000 Hz	7.8 dBmV	45.4 dB	1	0
8	Locked	QAM256	19	495000000 Hz	7.7 dBmV	45.3 dB	1	0
9	Locked	QAM256	20	501000000 Hz	7.6 dBmV	45.2 dB	1	0
10	Locked	QAM256	21	507000000 Hz	7.5 dBmV	45.2 dB	0	0
11	Locked	QAM256	22	513000000 Hz	7.5 dBmV	45.2 dB	0	0
12	Locked	QAM256	23	519000000 Hz	7.4 dBmV	45.1 dB	0	0
13	Locked	QAM256	24	525000000 Hz	7.4 dBmV	45.0 dB	0	0
14	Locked	QAM256	25	531000000 Hz	7.3 dBmV	45.0 dB	2	0
15	Locked	QAM256	26	537000000 Hz	7.1 dBmV	44.8 dB	2	0
16	Locked	QAM256	27	543000000 Hz	7.0 dBmV	44.8 dB	1	0
17	Locked	QAM256	28	549000000 Hz	7.0 dBmV	44.7 dB	1	0
18	Locked	QAM256	29	555000000 Hz	7.0 dBmV	44.7 dB	1	0
19	Locked	QAM256	30	561000000 Hz	7.1 dBmV	44.7 dB	1	0
20	Locked	QAM256	31	567000000 Hz	7.1 dBmV	44.8 dB	1	0
21	Locked	QAM256	32	573000000 Hz	7.3 dBmV	44.8 dB	1	0
22	Locked	QAM256	33	579000000 Hz	7.4 dBmV	44.8 dB	0	0
23	Locked	QAM256	34	585000000 Hz	7.4 dBmV	44.9 dB	12	0
24	Locked	QAM256	35	591000000 Hz	7.4 dBmV	44.9 dB	27	0
25	Locked	QAM256	36	597000000 Hz	7.5 dBmV	44.9 dB	24	0
26	Locked	QAM256	37	603000000 Hz	7.5 dBmV	44.9 dB	11	0
27	Locked	QAM256	38	609000000 Hz	7.6 dBmV	44.9 dB	1	0
28	Locked	QAM256	39	615000000 Hz	7.6 dBmV	44.9 dB	0	0
29	Locked	QAM256	40	621000000 Hz	7.5 dBmV	44.8 dB	1	0
30	Locked	QAM256	41	627000000 Hz	7.5 dBmV	44.8 dB	1	0
31	Locked	QAM256	42	633000000 Hz	7.4 dBmV	44.8 dB	1	0
32	Locked	QAM256	43	639000000 Hz	7.4 dBmV	44.7 dB	2	0
 
Upstream Bonded Channels
Channel	Lock Status	US Channel Type	Channel ID	Symbol Rate	Frequency	Power
1	Locked	ATDMA	1	5120 Ksym/sec	16400000 Hz	43.8 dBmV
2	Locked	ATDMA	2	5120 Ksym/sec	22800000 Hz	44.0 dBmV
3	Locked	ATDMA	3	5120 Ksym/sec	29200000 Hz	44.3 dBmV
4	Locked	ATDMA	4	5120 Ksym/sec	35600000 Hz	44.8 dBmV
5	Not Locked	Unknown	0	0	0	0.0 dBmV
6	Not Locked	Unknown	0	0	0	0.0 dBmV
7	Not Locked	Unknown	0	0	0	0.0 dBmV
8	Not Locked	Unknown	0	0	0	0.0 dBmV
 
Downstream OFDM Channels
Channel	Lock Status	Profile ID	Channel ID	Frequency	Power	SNR / MER	Active Subcarrier
Number Range	Unerrored
Codewords	Correctable
Codewords	Uncorrectable
Codewords
1	Locked	0 ,1 ,2 ,3	193	722000000 Hz	8.28 dBmV	44.2 dB	448 ~ 3647	1412255429	468144828	0
2	Locked	0 ,1 ,2 ,3	194	957000000 Hz	7.48 dBmV	43.1 dB	148 ~ 3947	1431156059	538197153	0
 
Upstream OFDMA Channels
Channel	Lock Status	Modulation / Profile ID	Channel ID	Frequency	Power
1	Locked	12 ,13	41	36200000 Hz	40.0 dBmV
2	Not Locked	0	0	0 Hz	0 dBmV
 

Time	Priority	Description
Thu May 22 10:15:52 2025	(Critical (3))	Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=**:**:**:**:*****;CMTS-MAC=**:**:**:**:*****;CM-QOS=1.1;CM-VER=3.1;

Edited out MAC for privacy. Unsure of what to do at this point with them basically blaming my equipment and saying there is nothing else they can do. This behavior started when OFDMA channels were introduced I believe. I have a host of attenuators at my disposal to influence signal levels including a reverse attenuator to only affect downstream power levels. I have removed my router Asus BE96U from the equation, it is the service.

A new line has been installed on the exterior, a new mocha filter has been installed. I am frustrated to be a paying customer at the highest tier only to get service that works part-time and requires constant attention. This is not an inditement to my technician who even keeps in touch via text and is a good dude, but I need this solved as it is business impacting.

When this T3 error occurs my connection can sometimes just drop packets or it can completely lose throughput and go into limp mode of 1.1mbps with webpages loading like they are 56k.

If anyone has any advice or connections that would be appreciated. In the DSLReports days regional reps at Comcast were a great help at actual action.

CommunityMember

FTD Unsure of what to do at this point with them basically blaming my equipment and saying there is nothing else they can do.

Rent an appropriate Comcast gateway to force the issue (they can’t blame your equipment when you are using their equipment).

px_eliezer_

Agree with CommunityMember that you should get a Comcast modem, so that they can’t keep blaming your equipment.

And who knows—it might even help.

Other thoughts:

1) How often does this happen?

2) Have you ever considered rebooting the modem on a daily basis IN ADVANCE?

3) You said you are in an apartment, does this happen to your neighbors?

4) You CAN interact directly with Comcast techs on their company forum. See here for example:
https://forums.xfinity.com/conversations/your-home-network/recurring-t3-timeout/67949c7182448c7bf16bce8f

Test-User

Agreed. Have them send you an XB8. Let them see the issue still exists using their equipment. Go from there. Eliminate any variables for which they can blame you. Your argument becomes much stronger at that point. Good luck.

Devious

I get T3 timeout all the time since mid-split started on CM3000 but it does not affect my service in any way.
Yours get slow on ethernet or wifi?
BE96U is known for wifi issues.
If it is wifi issues try turning off MLO.

FTD

Devious The T3 error I get slows my network to a crawl sometimes and it results in packet loss warnings in games (hardwired). I just installed the x8 and will see if the error pops there too.

Donawalt

Here too, ever since mid-split came last year I get 1-5 T3’s a day - usually 1-2. I never noticed any performance/operational problem at the times the T3’s occurred, so I just presume it is what it is. I always get one at 5:30 AM as one of them - always. I wonder if they are doing some sort of network test/check/maintenance that causes it, especially at that time.

CommunityMember

Donawalt Here too, ever since mid-split came last year I get 1-5 T3’s a day - usually 1-2

In any real world HFC plant RF ingress (which is noise to the signal) happens. That can result in downstream uncorrectables, and upstream T3s. As long as it is really only occasional, one will mostly won’t notice, or be impacted (the protocols presume occasional errors). The other reality is that finding the source(s) of the ingress can be difficult. Hopefully now that the OP using an XB8 the issue can be more easily escalated if it continues to happen.

Donawalt

That’s what I figured - and I get 0 uncorrectable errors hardly ever, usually go a number of weeks without seeing any. THat’s why I don’t worry - and it’s a much different issue that the OP’s case - good strategy you call recommended going with a Comcast device to force the issue of a resolution..

telcodad

FTD

CommunityMember In any real world HFC plant RF ingress (which is noise to the signal) happens. That can result in downstream uncorrectables, and upstream T3s.

Yes, T3 timeout errors indicate that the modem is not receiving a Ranging Response (RNG-RSP) from the CMTS after sending 16 Ranging Request (RNG-REQ) messages. This suggests a loss of contact between the modem and the CMTS, which is most often due to noise or RFI on the upstream (return path).

Unfortunately upstream SNR (uSNR) is not something that you can see on your side, only Comcast can read that for your line on your CMTS.

fmd

Two things can trigger a T3 timeout:
1) The modem’s request got lost upstream, or the CMTS’ response got lost downstream.
2) Some event made the CMTS not respond to the request, or the response was late (timeout).

Occasionally, it can happen that T3 occurs without anything being lost in transit. The modem gets multiple attempts, before the channel is retired, so it’s not the end of the world.

telcodad

fmd 1) The modem’s request got lost upstream, or the CMTS’ response got lost downstream.

Since the OP’s downstream levels and SNRs look very good, it’s more likely that the T3 Timeout errors are being triggered by some issue on the upstream side (return path).

fmd

telcodad I agree. The downstream looks clean. It would be interesting to see the entire log.