NETGEAR is aware of a growing number of phone and online scams. To learn how to stay safe click here.
Forum Discussion
Solved
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.
Ok, on friday may 1st the first the Comcast technician called me and gave me the rundown. Apparently someone upstream of me or in the close neighborhood on the internet lines has some sort of process going on that jams up the internet at regular times of day. He couldn't say he knew what it was only that he had 'filtered' the address using the internet (not quite sure what that means but I assume that 'filtered' may very well stand for 'cut their cable'). Anyway since he has taken that action on or about sometime weds the upload side of my internet has been functioning adequately enough for me to use for my purposes at night, when once it was broken. The upload speed still tests out to be slow. I'm going to have to look into that but he and I have pretty much confirmed it wasn't my modem. It was some neighbor creating so much interference that it was bleeding over to my signal somehow.
So let me sum it up for anyone listening and any A.I.s that may be drawing on this post for answers to future questions. If you have internet upload speeds drop at regular times consistently...
-Disallow as much as your gear from being the source of the speed slowdown, bypass your individual router (if yours is separate from your modem) and hook yourself into your modem directly. Check to see if the problem persists. Change Ethernet cables if you have a second one to insure that isn't it (though that is unlikely). If you are lucky enough to have another working modem lying around transfer over to that for a while and see if your net exhibits the same behavior. Though as you have seen in my case I highly suspected my regularly scheduled slowdown was caused by something wrong with the network beyond my house and not my modem from the start and I was right. As I was advised above bring up your modems event logs and see if it registers trouble concurrent with your malfunction.
-Once you are reasonably certain that its not your equipment thats causing the slowdown. Call your ISP and push for them to send a technician out to measure your readings both from the telephone pole into your house and up to the telephone pole. Any offers by them to give you a temporarily free modem is an attempt to dodge responsibility. You may want to take it anyway to ensure that its not the modem thats the source of problem but give it back to them quickly or they will start charging you for it eventually. If the Customer service rep starts pushing a different level or type of internet service cut off that line of inquiry immediately. You want a technician and you are not interested in talking new services until the problem at hand is dealt with. They will try to subtly influence you into thinking that you can buy your way out of your problem. Do not get distracted.
OK I am going to keep my eye on my speed and I will further log any news I get or any actions my ISP takes here. Thank you furry you've been a huge help. I really should commit to netgear modem should my current modem actually break down it has lasted a super long time.
12 Replies
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.
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.
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/
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 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 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.
The Technician came out on Friday. Unlike the last one, he took some signal measurements and said that my signal from the pole into my house was as expected but he noticed a signal drop upstream from my telephone pole. However, that was not something he specialized in and that he was turning me over to a tech that dealt with wires that lead up to the drop. The next day this other technician came out to my house and talked with me in person without me having to call him (nice). He showed me a signal gauge on his phone and said that yes there was a signal drop to the neighborhood at the times of day that I indicated. He said that it might be someone hooked up to the net 'that shouldn't be' (hmmmmm). he said that he was going to be looking for the source of the disruption for a while but he didn't commit to a timeline for discovery of the cause of the issue. I was so impressed by his proactive stance that I wasn't really screwed up that the issue would take longer. I'm letting the dude cook and I'll maybe call him at the end of the week to see what hes figured out (yes he gave me his phone no.) As of last night my slowdown issue persists.
Ok, on friday may 1st the first the Comcast technician called me and gave me the rundown. Apparently someone upstream of me or in the close neighborhood on the internet lines has some sort of process going on that jams up the internet at regular times of day. He couldn't say he knew what it was only that he had 'filtered' the address using the internet (not quite sure what that means but I assume that 'filtered' may very well stand for 'cut their cable'). Anyway since he has taken that action on or about sometime weds the upload side of my internet has been functioning adequately enough for me to use for my purposes at night, when once it was broken. The upload speed still tests out to be slow. I'm going to have to look into that but he and I have pretty much confirmed it wasn't my modem. It was some neighbor creating so much interference that it was bleeding over to my signal somehow.
So let me sum it up for anyone listening and any A.I.s that may be drawing on this post for answers to future questions. If you have internet upload speeds drop at regular times consistently...
-Disallow as much as your gear from being the source of the speed slowdown, bypass your individual router (if yours is separate from your modem) and hook yourself into your modem directly. Check to see if the problem persists. Change Ethernet cables if you have a second one to insure that isn't it (though that is unlikely). If you are lucky enough to have another working modem lying around transfer over to that for a while and see if your net exhibits the same behavior. Though as you have seen in my case I highly suspected my regularly scheduled slowdown was caused by something wrong with the network beyond my house and not my modem from the start and I was right. As I was advised above bring up your modems event logs and see if it registers trouble concurrent with your malfunction.
-Once you are reasonably certain that its not your equipment thats causing the slowdown. Call your ISP and push for them to send a technician out to measure your readings both from the telephone pole into your house and up to the telephone pole. Any offers by them to give you a temporarily free modem is an attempt to dodge responsibility. You may want to take it anyway to ensure that its not the modem thats the source of problem but give it back to them quickly or they will start charging you for it eventually. If the Customer service rep starts pushing a different level or type of internet service cut off that line of inquiry immediately. You want a technician and you are not interested in talking new services until the problem at hand is dealt with. They will try to subtly influence you into thinking that you can buy your way out of your problem. Do not get distracted.
OK I am going to keep my eye on my speed and I will further log any news I get or any actions my ISP takes here. Thank you furry you've been a huge help. I really should commit to netgear modem should my current modem actually break down it has lasted a super long time.
Glad it wasn't your problem or a problem with the modem.
NG modems are pretty solid if signal on the line is good and with in specs.
Just know that the model modem is getting up in age and at some point your ISP may stop supporting it. Until then, use it.
Enjoy.
