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user_3twg14
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08/31/2026 5:37 PM

Recurring internet drops with a customer-owned Arris S33 modem and need help investigating a DOCSIS connection issue.

I'm having recurring internet drops with a customer-owned Arris S33 modem and need help investigating a DOCSIS connection issue.

The same S33 has experienced these drops at two different Xfinity locations in two different states (Cambridge, MA and Knoxville, TN). I originally had an eero modem/router that experienced the same issue; I then replaced it with the S33 + TP-Link Deco mesh, and the drops continued.

The S33 is connected directly to the coax wall jack with no splitter.

The modem's downstream signal looks excellent (0–2 dBmV, 42–44 dB SNR, 0 uncorrectables), but the event log shows:

  • "UCD invalid or channel unusable"

  • "SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing"

  • Upstream diplexer changes between 5–42 MHz and 5–85 MHz

The upstream transmit power is approximately 46–47 dBmV.

The S33 is running firmware TB01.03.001.10_012022_212.S3, hardware V1.0.

I'd like to have an Xfinity technician check the CMTS-side diagnostics for this modem, particularly upstream ranging/T3/T4 errors, return-path noise, provisioning, and whether this is the current approved firmware for the S33.

Can Xfinity Support please review this and let me know if I should send a DM with my account information?

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8 days ago

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XfinityEmilyB

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9 days ago

 

user_3twg14 Welcome to our community forum! Thank you for reaching out so we can troubleshoot your Xfinity internet connection. What download and upload speeds are you currently subscribed to? Do you know if the infrastructure in your area has been upgraded? 

 

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user_3twg14

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Thanks. My subscribed speeds are up to 1 Gbps download and up to 35 Mbps upload.

I'm not sure whether the infrastructure at my current location has been upgraded.

However, the issue I'm trying to troubleshoot isn't related to my subscribed speeds. The problem is intermittent loss of internet connectivity. My Arris S33 is showing DOCSIS upstream errors, including:

  • "UCD invalid or channel unusable"

  • "SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing"

  • Upstream diplexer mode changes between 5–42 MHz and 5–85 MHz

The S33's upstream transmit power is currently approximately 46–47 dBmV, while the downstream signal looks very good (0–2 dBmV, 42–44 dB SNR, and 0 uncorrectables).

Could you check whether my location has been moved to an upgraded/mid-split upstream configuration and whether my S33 is correctly provisioned for the configuration Xfinity is currently using?

I'd also like to know whether you can see any upstream ranging failures, T3/T4 errors, return-path noise, or other indications on the CMTS side that the modem is losing upstream synchronization.

The same S33 has experienced this issue at two different Xfinity locations in different states, so I'm trying to determine whether this is a modem/provisioning issue rather than a local Wi-Fi or router issue.

(edited)

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9 days ago

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XfinityDemetrise

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Thank you for the info.  Sounds like I may need to take a closer look at your account.  Please follow the steps below to send a direct message. 

 

How to Send Us a Direct Message:

1.  Click "Sign In" if necessary.

2.  Click the "Direct Messaging" icon.

3.  Click the "Start new conversation" (pencil and paper) icon.

4.  In the "To:" line, type "Xfinity Support".

5.  As you type, a drop-down list will appear. Select "Xfinity Support" from that list.

6.  A "Xfinity Support" graphic will replace the "To:" line.

7.  Type your message in the text area near the bottom of the window.

8.  Press Enter to send it.

 

How to: Direct messaging within the forum | Xfinity Community Forum

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