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Have CM400 modem with upload slowdowns that happen at regular intervals
I have a CM400 modem and lately I have experienced a slowdown of only my upload speed starting at a specific time of day between 7:30pm and 8pm for the past week. My upload speed goes from 30mbps before that time to between an unusable 6 to 9 mbps after that the slowdown ends between 4 and 10am the next day (I wouldn't know I'm asleep then). Only to repeat the next day. My download speed remains the same acceptable normal speed its always been at all times of day. This has happened every day for the past week. I have directly connected my computer to the modem bypassing the router and the problem persists. I have contacted my ISP and they have said that my connectivity is fine and ofc they say my modem is to blame for the issue. They sent a repairman out. In addition to hard selling camcasts rental modem He helped replace some badly worn cable leading from the telephone pole to the house and I was hoping that would do it. but the following evening the problem returned and wil;l most liekly return tonight. I write to ask what could be causing the issue? Its an old router thats been serving me loyally for 7 years so maybe its time for it to go to the great modem recycler in the sky but I just wanted to ask here if they had every heard of such symptoms and if there had been a fix for it. Firmware problems? A virus that affects the modem only? some electronic noise that cuts in at only evening? If the modem speed was failing at random times I would just chalk it up to age and get a new one but the symptoms on their face look super suspicious. Thanks for any attn I get here and I can answer any questions.
9 Replies
I have made contact with Comcast several times through their customer service chat. I got through to one rep who seemed that she genuinely wanted to help. She issued me a repair ticket number that we could take this entire issue under and said that she would look into my symptoms. I waited a day and wrote them to ask how it was going and in that chat (with a different person)they said they would call me the next day .The next two days came and went with no call and I chatted with another person earlier today who tho I gave them the ticket no. seemed to have no new information and suggested, like all the other reps did, that I get a new modem. So to satisfy them, I did. I went out and got a newer and better modem. plugged it in earlier tonight so as to make sure that the new modem was active at the time the upload speed is dragging so that I could make a contrast with the old modem turned on the apps that was using to upload, did a speed test and guess what?
Problem persists.
Soooo I have another repair tech coming over tomorrow afternoon. He's prolly going to push the comcast modem. at least as a test and I will probably accept it. I genuinely don't know what else to do. They keep saying their net is fine and they even implied in todays call to hook up the newer modem that they are reaching the limits of what they are doing. They also don't do night calls with techs. So i can't demonstrate the flaw in progress. Oh well I guess I will try to explain it to the repair guy when he gets here tomorrow. Maybe he has an idea.
Try there modem out and see how it works, if the issue continues or not, then you'll know.
I shall alert my ISP to the Logs findings and copy the logs to them if that will help. I suppose that each time it says 'critical' or 'warning' in the log thats my ISP dropping out on me. I shall see what they say and get back to this thread with any reactions. Thank you.
I'm having to post the event log in two parts as the system won't allow me to post it in 1. Here is the second half. Kep in mind my upload problems began around 4/12...
Apr 13 00:10:26 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Apr 13 15:36:07 2026 Critical (3) No Ranging Response received - T3 time-out Apr 14 00:22:38 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Apr 14 02:41:43 2026 Critical (3) No Ranging Response received - T3 time-out Apr 16 12:58:20 2026 Critical (3) Unicast Ranging Received Abort Response - initializing MAC Apr 16 12:58:20 2026 Critical (3) No Ranging Response received - T3 time-out Apr 16 12:59:16 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Apr 16 12:59:19 2026 Critical (3) No Ranging Response received - T3 time-out Apr 17 09:35:36 2026 Critical (3) SYNC Timing Synchronization failure - Loss of Sync Apr 17 09:35:41 2026 Warning (5) Lost MDD Timeout Apr 17 09:36:25 2026 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out Apr 17 09:49:55 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Apr 17 22:02:38 2026 Critical (3) SYNC Timing Synchronization failure - Loss of Sync Apr 17 22:02:42 2026 Warning (5) Lost MDD Timeout Apr 17 22:03:31 2026 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out Apr 17 22:04:46 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Apr 17 22:04:49 2026 Critical (3) No Ranging Response received - T3 time-out Apr 17 22:05:46 2026 Warning (5) Initializing Channel Timeout Expires - Time the CM can perform initial ranging on all upstream channels in the TCS has expired Apr 17 22:05:47 2026 Warning (5) TCS Partial Service Apr 17 22:06:16 2026 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out Apr 18 01:28:30 2026 Critical (3) No Ranging Response received - T3 time-out Apr 18 15:02:17 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Apr 19 01:44:27 2026 Critical (3) No Ranging Response received - T3 time-out OK I'm at the modem webpage Connection status is 'good'. I am using firmware version V2.01.06 which I believe is the latest comcast applied firmware based on the first link you gave me. Event log is as follows...toms and if there had been a fix for it.
Mar 21 13:02:19 2026 Critical (3) No Ranging Response received - T3 time-out Mar 21 13:04:09 2026 Critical (3) SYNC Timing Synchronization failure - Loss of Sync Mar 21 13:04:15 2026 Warning (5) Lost MDD Timeout Mar 21 13:04:55 2026 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out Mar 21 13:06:15 2026 Critical (3) No Ranging Response received - T3 time-out Mar 21 13:06:30 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Mar 21 13:06:32 2026 Critical (3) No Ranging Response received - T3 time-out Mar 21 13:07:30 2026 Warning (5) Initializing Channel Timeout Expires - Time the CM can perform initial ranging on all upstream channels in the TCS has expired Mar 21 13:07:31 2026 Warning (5) TCS Partial Service Mar 21 13:08:00 2026 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out Mar 21 13:33:30 2026 Critical (3) No Ranging Response received - T3 time-out Mar 30 21:38:16 2026 Critical (3) SYNC Timing Synchronization failure - Loss of Sync Mar 30 21:38:21 2026 Warning (5) Lost MDD Timeout Mar 30 21:39:13 2026 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out Mar 30 21:40:23 2026 Critical (3) No Ranging Response received - T3 time-out Mar 30 21:40:40 2026 Warning (5) MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1 Mar 30 21:40:42 2026 Critical (3) No Ranging Response received - T3 time-out 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.
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://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/
My ISP is Xfinity/comcast.
Not sure what you mean by 'fully support'?
by 'connection status and event log page' I'm not sure how to get that. I have a couple friends who are more computer minded than I am so I'll get back to you with that info. I went to the second link you posted above AND '192.168...' link does not open because all my browsers call it a security risk.
You'll need to access the modems web page to gain access to the cable connections and event logs page.
Please allow the browser to access the web page. It's ok too.
https://kb.netgear.com/000036375/What-s-the-latest-firmware-version-of-my-NETGEAR-cable-modem-or-modem-router
Please post a copy and paste of the modems connection status and event log page.
https://kb.netgear.com/30007/How-do-I-obtain-the-cable-connection-information-from-a-NETGEAR-cable-modem-modem-router
Who is your ISP?
Does your ISP fully support the CM400 on there network? It's kinda old generation so maybe time to upgrade.
