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

I remember it is about -14dB, at work right now so I can't check real time, but it is around that number. Will check with another 12V power supply and see what's going on. Maybe wind blew out a cover somewhere and water get in.
No point in trying a different power supply or modem with -14db, there's a bad connection somewhere and if fiddling with those inside the premises doesn't help, and in the junction box (if they haven't put a tamper tag on it to show when it's been opened) then it's time to get them to send a tech out. Playing odds it is more likely outdoors (or in an attic above the insulation or under the home in a crawl space) where temperature and/or moisture fluctuates more.

That's unless it has consistently been near that low previously without the dropouts due to multiple splitters on the line, so is only dropping a little from the usual strength, then an amp could help, but it's still degraded below what it should be so the amp is something to think about after seeing what the signal strength comes up to after finding the fault.
 
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If anyone wants it, I've got an SMC D3CM1604, which is a 16x4 modem, it's just been sitting around for 3 years since I cancelled Comcast and went to AT&T, then when I went back to Comcast a year later I just rented their modem because I get unlimited data.
 
Blame game isn't always necessary. A field tech can hook their scanner-whatever up....
Just my experience with Spectrum. The reps are "helpful" initially then they notice I'm using my own modem and their tune changes. They quickly shift to offering to send a tech out "and if your modem is found to be the issue, you'll be charged the technician visit charge of (it was $150 last time I called and that was ~2 years ago)". They're actually eager to send someone out. I told them no each time and amazingly, the issue went away with no tech visit and I continued to use my modem.
 
^ I've got Spectrum, though in the former TWC, former Insightbb area. They've never billed me for a visit, never questioned my modem or router (besides the automated robot menu when you call, telling you to power cycle everything)... though a large part of that could be, it never was my modem or router to blame!

On the other hand no issue I ever had went away by itself, except known larger area outages.

They do question my cablecard boxes, like "why do you have that nobody has those anymore?" and are basically clueless to resolve cablecard issues with mine ceasing to work for no reason a couple times in the past ~18 mos. when they change something in their system (think last time it was moved more channels to SDV) till someone in the know called the Buffalo support center (1-866-532-2598) that knows how to do anything cablecard related, then they changed something on their system, not on my end.
 
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They do question my cablecard boxes, like "why do you have that nobody has those anymore?" and are basically clueless to resolve cablecard issues with mine ceasing to work for no reason a couple times in the past ~18 mos. when they change something in their system (think last time it was moved more channels to SDV) till someone in the know called the Buffalo support center (1-866-532-2598) that knows how to do anything cablecard related.
I believe the FCC no longer requires them to support cable cards.
 
^ True but they never mention requirement or lack thereof, just act like it's an alien creature, lol.

One of their rented set top boxes that my Samsung GX-SM530CF smart cablecard box replaced, literally had a cablecard in it that could be seen through the top vents, so as long as they use those boxes in their network I'm guessing their system has to still support them even if they deny new device, cablecard requests at some point.
 
^ True but they never mention requirement or lack thereof, just act like it's an alien creature, lol.

One of their rented set top boxes that my Samsung GX-SM530CF smart cablecard box replaced, literally had a cablecard in it that could be seen through the top vents, so as long as they use those boxes in their network I'm guessing their system has to still support them even if they deny allowing new device cablecard requests at some point.
The big thing that will kill most 3rd party set top boxes is that they rely on legacy STB OOB, Comcast is moving their boxes to being DOCSIS Settop Gateways, so if a thirdparty the box that lacks a DOCSIS modem built in can't communicate back to the headend then they'll probably just kill support for it.
 
When I had this problem at my last house (and after fighting Comcast to stop blaming my router and modem) it was caused by the drop from the pole having been eaten by squirrels.
 
On the other hand no issue I ever had went away by itself, except known larger area outages.
At least one instance we had ongoing stability issues over about a 4-week period. Others in the neighborhood FB group reported the same. I called at least twice during this period and they suggested my modem was to blame and I turned down their truck roll offer. When I called again, I got a friendly, helpful rep who told me they identified the issue somewhere down the street and the repair was scheduled within the week. He said their system actually identifies potential problems like this when clusters of modems report errors (T3, T4, timeouts, etc) so they don't have to wait for random customers to call in, if they even do, in hopes that someone connects the dots.
 
So I remembered wrong about the SNR, downstream is 32-24db, power upstream is 45-54dB but seems like channel 5 is only 1/2 the symbol rate and channel 4 is "Retries Exceed".

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Not sure if it stopped having problem after 3:24 because we didn't use it or if the problem is "fixed" on Comcast side.
 
So I remembered wrong about the SNR, downstream is 32-24db, power upstream is 45-54dB but seems like channel 5 is only 1/2 the symbol rate and channel 4 is "Retries Exceed".

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The symbol rate just means the width of the channel is 3.2mhz, while the others are 6.4mhz that has nothing to do with your signal levels. But with that low of levels and the weird power level distribution on the upstream channels with one channel screaming there's definetly something wrong, I'd go looking for loose or corroded connections, splitters, damage to the main line, if you can disconnect everything else from the cable line and connect the modem directly to the incoming line outside.
 
Not sure if it stopped having problem after 3:24 because we didn't use it or if the problem is "fixed" on Comcast side.
If I'm not mistaken, these errors will occur continuously just from the modem talking to the cable headend, whether you're using the internet or not. If you reboot your modem and watch the logs, those timeouts are likely occurring during boot-up too.
 
If I'm not mistaken, these errors will occur continuously just from the modem talking to the cable headend, whether you're using the internet or not. If you reboot your modem and watch the logs, those timeouts are likely occurring during boot-up too.
If it's in normal operation you shouldn't be seeing sync errors and T3s and T4s, if you see sync timing failures and T3s at boot that's normal but these don't appear to be happening during boot because it has a time, if it was during boot it'd say time not established.
 
What I see is T3 seems to be not that big of a problem (i.e. won't reboot) but T4 will lead to reboot. I don't mind being slightly slower but reboot is annoying.
 
Those downstream power levels are quite low, do you have any splitters anywhere before the modem?
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).

The bigger problem I see is the upstream channel 5, the symbol rate is lower, not sure if it has to do with interference or a bad part somewhere.
 
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The Xfinity Assistant Chat guys told me they have sent an "advance refresh signal" to my modem and ask me to reset (there is no such button, so I power cycle), saying it will help.

So far I left channel 5 on upstream and the downstream power level / SNR seems to have adjusted "slightly". Let's see how it goes.
 
It is a "Motorola" SB6121, which means it is older than 2013......
That is an old modem. I recently reworked my SB6141 which is newer than yours. Shine a light through the component's side of the case. Look for bulged out, leaky capacitors. I had eight of nine electrolytic capacitors leaky and bulged out. It was no big deal for me to replace these and restore good operation of my 6141. If you see this, it's time to replace your old modem with a DOCSIS 3.1 compliant unit and proceed from there.
 
2 to 3 splitter (2 on 1 side 3 on the other, like a trapezoid shape)
That would be a 4:1 splitter. Make sure it is installed correctly with the incoming cable going to the port labeled "IN", which is usually the center one on the side with 3 ports.

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. If you only have Internet service, remove the splitter and replace it with a coupler directly to the modem line, leave the other coax in the house disconnected. It's also a good idea to test by connecting the modem directly to the line from the street, like you would with a landline phone.

Likely there is a problem in the cable drop from the street, or they need to re-evaluate and adjust levels on your block. 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.
 
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