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Intermittent connection drops during calls; modem log shows T3 timeouts, sync loss, and downstream uncorrectables
Hi Xfinity,
I’m having an intermittent connectivity/stability issue, not a simple speed issue. The connection briefly drops in and out, which is enough to disrupt voice/video calls. When the service is up, routine checks can still look normal, so this seems more like a reliability problem than a bandwidth problem. Xfinity’s own troubleshooting flow also distinguishes between general connection checks and cases that may need deeper troubleshooting or a technician if warranted.
I pulled the diagnostics from my modem, and a few things stand out:
Repeated critical log entries for “SYNC Timing Synchronization failure - Loss of Sync"
Repeated critical log entries for “No Ranging Response received - T3 time-out”
A noticeable number of downstream uncorrectable codewords on multiple channels, including 483, 531, 573, 591, 609, 615, 651, 669 MHz
Downstream power is roughly +2.0 to +6.3 dBmV with SNR around 37.3 to 37.6 dB, and the OFDM downstream channel shows +7.0 dBmV and 38.4 dB MER.
Upstream bonded channels are around 48.5 / 49.8 / 50.5 / 50.5 dBmV.
For reference, NETGEAR says good downstream power is -7 to +7 dBmV and good upstream power is 38 to 48 dBmV. My downstream snapshot seems to look okay, but upstream transmit power appears to be at or slightly above that guidance on several channels.
I also found Xfinity forum discussions where recurring T3/T4 timeout patterns with otherwise decent downstream readings were treated as possible upstream noise, return-path, drop, or plant-side impairments.
Could someone kindly review this for:
line stability / disconnect history over time
any upstream or return-path impairment
drop, connector, splitter, grounding block, or tap issues
any area impairment that could match intermittent brief outages
I’m hoping for a review beyond a single “signals look okay right now” snapshot, since the issue is intermittent and the modem log does show repeated T3 timeouts, sync-loss events, and downstream uncorrectables.
Thank you.
XfinityShawn
Official Employee
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2.3K Messages
17 hours ago
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EG
Expert
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116.9K Messages
2 hours ago
@user_behrf3 Things you can check and see if they apply;
In an effort to try to obtain better connectivity / more wiggle room, check to see if there are any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/reconfigured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage from Home Depot, Target, Wal-Mart, etc. Splitters should be swapped with known to be good / new ones to test.
Also, check the coax cable for any damage such as cuts, nicks, abrasions, kinks, sharp bends, or animal chews.
If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two-way splitter connected directly off of the drop from the street / pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.
Good luck with it !
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