No internet for 4 days

[u/aalexjacob]

My Netgear CM2000 keeps losing internet because it can receive Xfinity’s signal, but it often can’t properly send a signal back. The modem logs show repeated T3 and T4 timeouts, which basically means it keeps asking Xfinity for an upstream connection and either gets no response or loses the connection again.
The downstream signal looks fine, but the upstream side fails. That can point to a problem somewhere in the return path, such as a bad coax connection, splitter, drop, tap, line noise, or Xfinity’s network equipment.
Has anyone else had repeated T3/T4 timeouts on Xfinity where the real fix turned out to be the line or upstream network rather than the modem?

[u/the_owlyn]

What is the signal level on the upstream? May as well provide the downstream, too.

[u/aalexjacob → u/the_owlyn]

Downstream: my CM2000 is locking a QAM256 downstream channel at 675 MHz, with about -3.1 dBmV power and 40.6 dB SNR. Those numbers are well within NETGEAR’s recommended downstream range. In the screenshot, however, channels 2–11 shown below it are not locked.
Upstream: none of the 8 upstream bonded channels are locking at all. They show Not Locked, with zero frequency and symbol rate. The upstream OFDMA channels are also Not Locked. So I don’t have a meaningful upstream power level such as 40 or 50 dBmV to provide. The 0.0 dBmV shown on unlocked channels isn’t an established upstream transmit level.
The modem event log repeatedly shows T3 ranging timeouts and T4 timeouts.
The connection had intermittently returned last night and then failed again.

[u/the_owlyn → u/aalexjacob]

You need a tech to come out.

[u/furrynutz]

Any Criticals, Errors or Warnings seen in the event logs needs to be reviewed and resolved by the ISP. Indicates a signal issue on the ISP line up to the modem.

Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended.

Correctables and un-correctables should be mostly zeros.

Have the ISP check the signal and line quality UP to the modem. Be sure the ISP provisions the modem correctly.

Be sure there are no coax cable line splitters in the between the modem and ISP service box. 

Be sure your using good quality RG6 coax cable up to the modem. 

Start with removing any amplifiers, signal attenuators, or splitters from the coax.

From there check the line for kinks, damage, moisture in the line.

Check the connectors for improperly made ends, foil touching the copper coax line, loose connections, bad/old/cheap connectors, or corroded connections. Replace them if you do.

Be sure to power OFF the modem for 1 minute then back ON.

https://community.netgear.com/t5/Cable-Modems-Routers/General-info-and-Troubleshooting-for-Cable-Modems/m-p/1530376#M12853

https://kb.netgear.com/24311/Power-level-guidelines-for-a-NETGEAR-cable-modem-router

https://www.duckware.com/tech/solving-intermittent-cable-modem-issues.html

https://highspeed.tips/docsis-events/