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It would be interesting to see if you still get those T3 or T4 timeouts while connected to the main line bypassing the splitters and your in- house wiring. That would at least rule out a big chunk of the puzzle.
Having the modem behind a 2 way and a 3 way splitter did indeed degrade the signal too much, as you discovered by moving the modem to the main line and behind the 2 way to test. Did you also replace the coax in the walls, attic, crawlspaces? Sounds like you might have a hungry squirrel.
Your cable swapping took you in the wrong direction, and you have reversed the definitions of upstream and downtream. 'Better' is to make the upstream (US or transmit) value lower (ALWAYS below +51) and downstream (DS or receive) level to be higher (within a range of -8.0 min to +10.0 max).
The modem should always be connected as close as possible to the incoming drop cable signal, preferably using no splitter, but a single 2-way split between modem and TV is usually OK. Moving your modem's cable to the verry first 2-way splitter was the correct change to make.
A T3 error message says that your modem stopped receiving the downstream signal for up to three seconds, while the T4 error messages mean the outage was longer than 3 seconds, forcing the modem to reset and scan again for the downstream channels, possibly with a new channel list.
To fix T3 and T4 errors requires checking the cable path from the modem back toward the node and CMTS. Make sure all your indoor cable connections are more than just finger-tight and not wet or corroded. Also be sure the grounding block is connected to the powerline ground near yuour electric meter. The Spectrum home service tech will check the ones on the drop cable outdoors, whether it is buried or overhead.
I was able to locate your account based on your registration information. Current signal levels are good and from our diagnostics, I can confirm that the current uptime (without an interruption) is the longest it has been in six weeks. If you see the issue return, then we recommend a service call. To set up a service call, you can contact us by private message @Forums_Help or you can contact our Social Media Customer Care team: