Repeat Xfinity Line Failure – Prior Tech Test Failed, Now T3 Timeouts/Loss of Sync and 47–51 dBmV Upstream Even at Demarc

I recently restarted Xfinity 1 Gig service after previously canceling it because the connection was unreliable. During my first attempt with Xfinity, I used the original Xfinity gateway. A field technician came out, told me the line test failed, and said another technician was supposed to come back to correct the outside issue. That follow-up never happened. After eight days of poor service, I canceled Xfinity.

I am now giving Xfinity another try, this time using my own ARRIS SURFboard SB8200v3 modem and an ASUS TUF Gaming BE9400 router, with T-Mobile Home Internet configured as an automatic failover WAN. The same type of instability is occurring again. Given that the original Xfinity gateway had the problem, the prior field technician documented a failed line test, and the current SB8200v3 shows the same symptoms even when tested directly at the exterior demarcation point, this does not appear to be a modem, router, Wi-Fi, splitter, or inside-coax problem. Everything I have tested points back to the Xfinity drop, tap, return path, or local plant.

When the Xfinity connection fails, the ASUS detects the primary WAN as unavailable and automatically fails over to T-Mobile. The failover itself works correctly.

The SB8200v3 event log shows repeated DOCSIS errors, including:

• No Ranging Response received - T3 time-out
• SYNC Timing Synchronization failure - Loss of Sync
• Lost MDD Timeout
• MDD Recovery following MDD Loss
• RNG-RSP CCAP Commanded Power in Excess of 6 dB Below the Value Corresponding to the Top of the DRW
• DBC-ACK not received

One significant event occurred on September 14 around 8:59 AM:

• 08:59:04 – SYNC Timing Synchronization failure - Loss of Sync
• 08:59:11 – Lost MDD Timeout
• 08:59:13 – MDD Recovery following MDD Loss
• 08:59:17–08:59:20 – multiple T3 timeouts and ranging/power warnings

Additional T3 timeouts occurred at:

• Sep 12 19:13
• Sep 12 23:25
• Sep 13 18:43
• Sep 14 08:59
• Sep 14 13:59
• Sep 14 20:30

The ASUS router has also recorded the Xfinity WAN going down and later recovering. During one recovery, it temporarily received a 192.168.100.x address from the modem while the modem was coming back online, then eventually received the normal public Xfinity address and restored service.

Signal levels in the normal indoor modem location

Downstream:

• Lower-frequency channels: approximately -2 to -3 dBmV
• Higher-frequency channels: approximately -7 to -9 dBmV
• SNR/MER: 36–38 dB
• SC-QAM uncorrectables: 0

Upstream:

• 48 dBmV
• 49 dBmV
• 51 dBmV
• 49 dBmV

Splitters, coax, and inside wiring have already been eliminated as likely causes

There are no coax splitters at all between the Xfinity drop and the modem.

The normal signal path is:

Xfinity drop from roadside pedestal → grounded coax block/demarc on side of house → single coax run into house → SB8200v3

There are no TVs, MoCA adapters, amplifiers, additional outlets, or other devices sharing the coax.

I have also inspected the accessible coax run and fittings for obvious physical problems. There are no known cuts, nicks, abrasions, animal damage, sharp bends, or unnecessary connections.

The fittings at the house demarc have been checked for tightness and obvious corrosion/oxidation.

Because there are no splitters to remove or reconfigure, I went a step further and completely eliminated the house coax run from the test.

Direct demarc test

I disconnected the house-side coax at the exterior demarc and connected the SB8200 directly at the demarcation point, eliminating the entire inside coax run from the signal path.

After allowing the modem to fully lock and become operational, the readings were:

Downstream at the demarc

• Lower-frequency channels: approximately -1 dBmV
• Higher-frequency channels: approximately -5 to -6 dBmV
• SNR/MER: 36–38 dB
• Uncorrectables: 0

Upstream at the demarc

• Channel 17, 16.3 MHz: 51 dBmV
• Channel 18, 22.8 MHz: 48 dBmV
• Channel 19, 29.2 MHz: 47 dBmV
• Channel 20, 35.6 MHz: 48 dBmV

For comparison, the normal indoor upstream readings were:

48 / 49 / 51 / 49 dBmV

So eliminating the entire house coax run changed the upstream levels by only about 0–1 dB.

That effectively rules out splitters, the interior coax run, and normal in-home attenuation as the cause of the high upstream levels.

The downstream signal improved slightly when testing at the demarc, as expected from removing cable length, but the upstream remained essentially unchanged.

Why I believe this is an Xfinity-side issue

This is not the first time this connection has failed.

On my previous Xfinity installation:

• I was using the Xfinity-supplied gateway, not my current SB8200v3.
• An Xfinity field technician came to the property.
• The technician told me the line test failed.
• I was told another technician was supposed to return to address the outside issue.
• That follow-up never happened.
• After approximately eight days of unreliable service, I canceled Xfinity.

I have now restarted service with entirely different customer equipment and am seeing the same basic problem.

At this point I have reproduced the issue with:

• the original Xfinity gateway during the previous installation
• my current SB8200v3
• my current ASUS router monitoring the WAN
• the modem in its normal indoor location
• the modem connected directly at the exterior demarc with the house coax removed

The common denominator is the Xfinity network upstream of my demarcation point.

What I am asking Xfinity to investigate

I would like a field technician to inspect and test:

• The drop between the roadside pedestal and my house
• The connector/termination at the pedestal
• The tap
• The Xfinity-side connection at the demarc
• Upstream/return-path levels
• Return-path noise or ingress
• Any local plant issue that could explain the repeated T3 timeouts, Lost MDD events, Loss of Sync, and intermittent service interruptions
• The previously failed line condition that was apparently never corrected during my first installation

The modem can look normal between failures, but the DOCSIS event log, upstream levels, actual WAN outages, prior failed field test, and direct-demarc testing all point to an intermittent outside-line problem.

Please do not suggest I call back to Xfinity's offshore phone support. My experience with the India-based customer support line has been extremely poor and it has not been capable of addressing this issue. Repeating scripted modem resets, reprovisioning, Wi-Fi troubleshooting, checking for splitters, or telling me to tighten indoor coax connections is not a solution at this point. It is actually insulting frankly, I've been in IT for over 20 years. Whatever that was I just dealt with is not customer support.

I have already:

• Had this same problem previously using the Xfinity-provided gateway
• Had an Xfinity field technician tell me the line test failed
• Waited for a promised follow-up technician who never came
• Canceled the original service after eight days because the problem was not corrected
• Restarted Xfinity service using completely different modem/router equipment
• Verified there are no splitters
• Checked the accessible coax and fittings
• Captured repeated DOCSIS T3, Lost MDD, and Loss of Sync events
• Verified upstream levels reaching 51 dBmV
• Tested the modem directly at the exterior demarc
• Completely eliminated the inside coax run from the test
• Confirmed that upstream levels remain essentially unchanged at 47–51 dBmV even at the demarc

I am specifically requesting assistance from an Xfinity employee who can arrange a field technician visit to investigate the drop, tap, upstream return path, and local plant rather than repeating basic customer-premises troubleshooting.

So please, can an Xfinity employee here please help arrange that visit here? I am literally begging anyone who works for Xfinity to help me out at this point. It can't possibly be this hard. Again, please. I am here begging for support and giving all the possible information I can to help you. The Customer Support people are not a solution, they are why people cancel. Please give me an American phone number I can call through DMs, email me it if its not supposed to be public, or work with me to schedule someone to come out. Please.