Cable modem reboot every 20 mins due to T3 and T4 timeout

Splitting the signal 4 ways is a loss of 7 dB to each output. If you really need a splitter at the house because there are 2 or 3 active TV boxes plus the modem, the cable company should have increased your power from the street to compensate.
That's not how they do it. if they do something to adjust the signal levels at your home that would effect everyone else, the system is designed to have certain power levels distributed at certain points, The OP needs to check for bad connections or chewed up cables, -13db even after going through a 4 way split, I'm sure is a bit lower than intended by Comcast's system design.
 
I know that at the side of my house, where the Comcast signal cable comes into the interior, it has a 2 to 3 splitter (2 on 1 side 3 on the other, like a trapezoid shape), then after that one of them goes to the wall outlet and I use a cable from the wall outlet directly into my modem, no splitter in between).
Do you use them for internet only or TV and internet ? We only have internet from the cableco but the box on the side of the house had splitters feeding other splitters and about 5-6 cables going into our house. I eliminated all of the splitters. If you have TV, this might not be an option for you though you might be able to get rid of some splitters.
 
I only use internet, and I can try to find a coupler to see if it "fix" the issue. However at the moment I just made an appointment to pick up a Comcast modem and see if it 'diagnose' correctly afterward, and if not then they need to come out. If it works with their modem but not mine, I'll get a new one from BestBuy and return the Comcast modem. $14 for 1 diagnostic is not too bad.

I'll check if there are any chewed up cable but most likely it is moisture somewhere. There was a storm a couple days ago and I wouldn't be surprised some water get into the local lawn box.
 
If you've already replaced bad capacitors in the modem then, it's could be the cable from the tap to your junction box that needs replacing by the cable company. Typically though the modem will start to lose some downstream channels before rebooting. If Comcast is like Spectrum, they won't do this until you get 'their' modem and it still drops connection at which point they'll send a tech who will finally confirm it's not the modem. Or maybe the Spectrum CS by me sucks more than average. Good luck!
 
If you've already replaced bad capacitors in the modem then, it's could be the cable from the tap to your junction box that needs replacing by the cable company. Typically though the modem will start to lose some downstream channels before rebooting. If Comcast is like Spectrum, they won't do this until you get 'their' modem and it still drops connection at which point they'll send a tech who will finally confirm it's not the modem. Or maybe the Spectrum CS by me sucks more than average. Good luck!
Yeah, but if the problem is from inside the house, well anything past the ground block outside, then Comcast will charge you for the privilege of a truck roll.
 
But it is possible these low signals and rebooting (which I didn't think a modem would be programmed to reboot because of signal loss) are indeed a faulty modem. Anyone using their own modem should keep a spare modem on hand for troubleshooting.
From the logs that appears to be exactly what it's doing, like log entry "Cable Modem Reboot Due to T4 timeout" has to be something programmed into it to do like if it loses signal for ~30 seconds, not just an instability causing a crash.

At the same time, I like to replace my modems as preventative maintenance every few years. When I retired my SB6121, still working fine with no outages, I noted the stats to compare to the new modem replacing it. Here's that, though these downstream signal levels are with an amp on the line which increased it around 8db:

sb6121.png
 
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That's not how they do it. if they do something to adjust the signal levels at your home that would effect everyone else, the system is designed to have certain power levels distributed at certain points, The OP needs to check for bad connections or chewed up cables, -13db even after going through a 4 way split, I'm sure is a bit lower than intended by Comcast's system design.
Depends on how you want to word it. Here, there is a series attenuator on every run to individual residences. If the equipment is already being attenuated further by splitters, they can decrease that attenuation to increase the signal strength, though it is a prime example of a reason to get an amp, except something else is also *probably* causing further signal loss intermittently like a bad cable connection.

It would have helped to know what the downstream signal level was long before these events started, so we had a baseline of how much it's dropped since then during the outage events.
 
1636591118341.png


Rented a Comcast modem / router and so far in the last 15 mins this is what I have. Seems like the power level are "similar" for most downstream and upstream channels, the SNR on downstream gone from about 33 to 38-39 dB.

This new modem doesn't have a log that I can use to check if there are T3 or T4 timeout. I guess I have to use it for a few days to see for sure. The Xfinity store guys say I can return it and the rent would be prorated.
 
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Rented a Comcast modem / router and so far in the last 15 mins this is what I have.
The signals are still low, I'm interested in all the "not locked channels that are 40db lower in power level that are reported, is that just useless garbage or is it possible you still have an old internet only filter on your line that blocked channels 2-68 back in the day because that's what it looks like looks like the channel at 495mhz which would be cable TV channel 69 is showing signs of rolloff from the 2-68 filter.
 
Tell Comcast support you suspect there's an old filter from the old days when they would use those filters and they'll send a truck out to inspect and remove it if that's the case because in the all digital encrypted system they don't use those anymore and they interfere with the DOCSIS frequencies they use.
 
Tell Comcast support you suspect there's an old filter from the old days when they would use those filters and they'll send a truck out to inspect and remove it if that's the case because in the all digital encrypted system they don't use those anymore and they interfere with the DOCSIS frequencies they use.
I will try that. Will they still send a truck out if I am getting the speed they advertise for?
 
I had a sb6121 back in the day but then upgraded to the sb6183 but currently using a Netgear from Costco now since I've upgraded to gigabit the old modem was a bottleneck and next was my Ubiquiti router that I also had to replace. A long time ago I had issues with freezing rain, Comcast came and supposedly replaced those connectors on all of the poles that are on my street and I haven't had any issues since and that's been several years ago.
 
If the OP wants it, for the cost of shipping I can send him an SMC D3CM1604, I don't know what I've done with the adapter, but any 12V adapter will work, a small flat rate envelope should be big enough to fit it, I'll just make sure I package around the cable connector doesn't get broken in shipment, but for $10 that's a pretty good upgrade, it's a broadcom chipset in the modem, should be identical to the one used in the Netgear CM500 and SB6183.
 
Thanks for the offer. Currently, I am "renting" a Comcast modem and I just got the Xfinity tech to send out a truck tomorrow. The new Technicolor "gateway" they gave out is loud (with active fan cooling) and it is pretty repeatable that downstream channels below 19 have power problems locked or unlocked.
 
I meant once you get everything fix, have you called or used online chat and got it setup for someone from Comcast to come out and take a look and remove the filter that appears to be on your line, if you see them while they're doing it you can eaily get them to hand you a splice barrel so you can bypass all the splits in your house.
 
Cable guy just came, took out the old filter and the 1 to 4 split, re-crimp every single coax connector with a new one. Signal strength went from -54 / -9.2 to -3.1 / 0.9

I think this is good now. I will wait for a few more days and see if it is stable, then switch to my own modem.


DownstreamChannel Bonding Value
Index3217181920212223242526272829303133343536373839404142434445464748
Lock StatusLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLockedLocked
Frequency579 MHz483 MHz489 MHz495 MHz507 MHz513 MHz519 MHz525 MHz531 MHz537 MHz543 MHz549 MHz555 MHz561 MHz567 MHz573 MHz585 MHz591 MHz597 MHz603 MHz609 MHz615 MHz621 MHz627 MHz633 MHz639 MHz645 MHz651 MHz657 MHz663 MHz669 MHz850000000
SNR39.0 dB39.8 dB39.9 dB39.9 dB39.6 dB39.7 dB39.9 dB39.7 dB39.5 dB39.2 dB39.3 dB39.6 dB39.6 dB39.3 dB38.9 dB38.8 dB38.9 dB38.9 dB38.5 dB38.2 dB38.5 dB38.6 dB38.2 dB38.1 dB38.1 dB38.4 dB38.5 dB38.2 dB38.2 dB38.0 dB38.3 dB36.6 dB
Power Level-2.1 dBmV0.7 dBmV-1.0 dBmV-1.2 dBmV-1.4 dBmV-1.2 dBmV0.9 dBmV-1.1 dBmV-1.4 dBmV-1.7 dBmV-1.7 dBmV-1.3 dBmV-1.4 dBmV-1.6 dBmV-2.2 dBmV-2.2 dBmV-2.3 dBmV-2.6 dBmV-3.1 dBmV-3.3 dBmV-3.0 dBmV-2.9 dBmV-3.1 dBmV-3.4 dBmV-3.4 dBmV-2.9 dBmV-2.7 dBmV-2.7 dBmV-2.6 dBmV-2.7 dBmV-2.6 dBmV-2.4 dBmV
Modulation256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAM256 QAMOFDM
 
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