Recurring T3/T4 Timeouts, SYNC Failures, and UCD Errors Since August — NETGEAR CAX80 / Xfinity

I'm looking for some technical assistance with recurring connectivity problems on my Xfinity internet service. I've been experiencing intermittent internet interruptions, packet loss, and latency spikes that interfere with working from home, especially when using a corporate VPN.

I'm using a NETGEAR CAX80 DOCSIS 3.1 cable modem. I initially suspected my home networking equipment, but after reviewing several months of cable modem diagnostic logs, I'm increasingly concerned about a possible intermittent DOCSIS signal or communication issue.

The important detail is that these errors have been occurring since at least August 2026 and are continuing in November.

Equipment and Symptoms

• ISP: Xfinity/Comcast

• Internet plan: 1.2–2 Gbps tier

• Modem: NETGEAR CAX80 (modem-only mode)

• Router: UniFi Express 7 (UX7)

• Symptoms: Intermittent packet loss, internet interruptions, latency spikes, and work VPN connectivity problems

My UniFi gateway has independently reported WAN packet loss, high latency, and an approximately 43-minute internet reachability interruption on November 3.

Critical Errors Found in My CAX80 Logs

August 23, 2026

The modem recorded repeated clusters of:

• No Ranging Response received - T3 time-out

• Started Unicast Maintenance Ranging - No Response received - T3 time-out

• UCD invalid or channel unusable

These errors were occurring repeatedly within seconds of each other, including around 10:35–10:36 AM and again during the evening around 6:52 PM.

August 26, 2026

I captured another series of critical errors, again showing repeated T3 timeouts and UCD invalid/channel unusable messages. Several occurred within seconds of one another around 7:14 AM.

November 5–6, 2026

The problem continued with repeated T3 timeouts and frequent DOCSIS CM-STATUS messages involving downstream OFDM channels 193 and 194, particularly Event Type Codes 16 and 24.

On November 6, T3 errors were recorded at 6:14, 6:36, 6:42, 7:01, 8:00, 8:25, and 8:32 AM.

November 6 — New T4 Timeouts and Synchronization Failures

A later diagnostic capture revealed more serious errors around 10:12 AM:

• Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out

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

• UCD invalid or channel unusable

The log shows multiple T4 timeouts and synchronization failures within a short period.

The modem subsequently recorded downstream profile assignment changes during initialization around 10:27 AM. It also contains entries dated January 1, 1970, suggesting its system clock had not yet synchronized during part of the startup process.

This appears consistent with the modem losing communication and subsequently going through reacquisition or initialization, although I would appreciate help determining the actual cause.

What makes the issue frustrating is that the CAX80's diagnostic interface consistently reports its connection status as "Good."

Recent readings include:

Parameter	Readings
Downstream QAM power	Approximately +5.6 to +9.6 dBmV
Downstream SNR	Approximately 41.6–42.7 dB
Upstream power	Approximately 39–40.3 dBmV
Downstream channels	32 QAM channels locked
Downstream OFDM	2 channels locked
Upstream channels	4 ATDMA channels locked
Reported uncorrectables	0 in the captured tables

The OFDM channels have substantial correctable codeword counts, but no reported uncorrectable codewords.

These readings seem reasonable when the modem is online, yet the critical errors keep returning.

Given the combination of repeated T3 timeouts, T4 timeouts, synchronization failures, and UCD invalid/channel unusable events over several months, I'm wondering whether this points toward an intermittent issue with the coax drop, upstream noise or ingress, an outside plant problem, or something at the CMTS.

Specifically:

1. Could intermittent upstream noise or ingress cause repeated T3 and T4 timeouts while the modem's current upstream transmit power and downstream SNR remain normal?

2. What would explain the repeated UCD invalid/channel unusable events occurring alongside the T3 timeouts?

3. Are the T4 timeouts and synchronization failures on November 6 indicative of an actual loss of DOCSIS connectivity?

4. Can Xfinity examine historical upstream receive MER/SNR, ranging failures, and other CMTS-side diagnostics for my modem?

5. Could the repeated OFDM CM-STATUS Event Type 16 and 24 messages indicate a separate downstream impairment that should also be investigated?

6. Would checking the drop, connectors, ground block, and tap help identify the issue, or should this be escalated for node/plant investigation?

I have another Xfinity technician scheduled for Saturday, November 7. This will be the 3rd visit. The first saw the issues coming from the street and made a phone call and said it would be fixed in a few days. The one that came today didn't even get the correct information from dispatch and said that my signals look good and then left. 

My concern is that the technician may arrive when the modem's signal levels are normal, run a speed test, and find no obvious problem.

Since I've documented repeated critical DOCSIS errors in August and November, I'd like to make sure the issue is investigated beyond a single point-in-time signal reading.

I'd particularly appreciate input from anyone familiar with DOCSIS 3.1 diagnostics or Xfinity's CMTS-side monitoring tools.

Does the combination of recurring T3 timeouts, T4 timeouts, UCD errors, and synchronization failures suggest a particular area of the cable network that should be investigated first?

Thanks in advance for any guidance.