Hi, I have a CM400 modem that I've had and actively used for nearly 3 years without issue. Suddenly, yesterday (and today), my speeds were painfully slow, clocking in at 1.5 - 2.5Mpbs downlink (normally 150Mpbs down). I've tried doing a power cycle, and it helps for at most a few minutes. Running a speed test within minutes of a power cycle will yield my normal 150Mpbs downlink connection. I've contacted my cable company (Cox) and they ran some tests on their side and said there is nothing wrong with the network and that their testing to my modem shows that there is some interal failure causing the issue. As usual, with cable companies, I'm a bit of a skeptic so I wanted to see if anyone here could offer me any insight or advice on what to do or try to fix the issue? This has never happened before and it hasn't improved since last night thoughout today with the exception of the few minutes each time I power cycle.

Thanks for the insight and tip! It seems I was able to solve the issue after isolating the problem to the router. Hardwire connection to the modem gave me the proper speeds (I had hard connected through the router previously, doh!). Unplugging both modem and router for 15 minutes and then getting them back online seems to have done the trick...heading into almost an hour with a stable fast connection now. Hopefully this was all that was needed! Thank you!

CM400 Router suddenly very slow, briefly fast after a reset

5 Replies

plemans
Guru - Experienced User
May 21, 2020
xLyrical wrote:

I've contacted my cable company (Cox) and they ran some tests on their side and said there is nothing wrong with the network and that their testing to my modem shows that there is some interal failure causing the issue. As usual, with cable companies, I'm a bit of a skeptic so I wanted to see if anyone here could offer me any insight or advice on what to do or try to fix the issue? This has never happened before and it hasn't improved since last night thoughout today with the exception of the few minutes each time I power cycle.
Very aptly put. They're very good at blaming 3rd party equipment.
to start with, if you have any splitters, attenuators, or amplifiers in the coax line. remove them. Also, directly connect the modem right where the coax enters the home. This prevents devices in line or the coax within the walls from being the issue.
Use it for a bit (however long it took to drop speeds). If they drop again, log into the modem and go to the cable connection page and get a screen snip of that and your logs. That shows us the connection back to the isp. I'd also recommend just directly connect a pc to the modem for this so you can be sure it isn't a router issue causing the slow downs.
- xLyrical
  Tutor
  May 21, 2020
  Thanks for the insight and tip! It seems I was able to solve the issue after isolating the problem to the router. Hardwire connection to the modem gave me the proper speeds (I had hard connected through the router previously, doh!). Unplugging both modem and router for 15 minutes and then getting them back online seems to have done the trick...heading into almost an hour with a stable fast connection now. Hopefully this was all that was needed!
  
  Thank you!
  - Bwheat24
    Initiate
    May 26, 2020
    Did you get this actually solved? Having the same issue for a month and I'm with Xfinity. They said the same thing

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CM400 Router suddenly very slow, briefly fast after a reset