It’s the classic post-update nightmare: the installation finishes, the Mac reboots, but one of your screens stays pitch black. In my case, it was a Dell U3219Q connected via USB-C to an M2 Mac Mini. The strange part? The mouse and keyboard plugged into the monitor’s USB hub were working perfectly. Only the video signal was missing.

Here is the step-by-step log of a deep dive into the macOS system logs to find the culprit.

The Situation

• Hardware: Mac Mini M2, Dell U3219Q (via USB-C).
• Symptom: „No USB-C Signal“ on the monitor. However, USB data (hub) was still being passed to the Mac.
• Software: macOS 15.4 (Build 26.4.1).

Looking Under the Hood (Terminal Analysis)

To see if macOS was even attempting to talk to the monitor on a video level, I used the Unified Logging System. This command allows you to watch the „handshake“ between the Mac and the monitor in real-time:

log stream --predicate 'subsystem == "com.apple.Skywalk" OR eventMessage CONTAINS "DisplayPort" OR eventMessage CONTAINS "Thunderbolt"'

The „Failure“ Log (Excerpt)

When plugging in the cable, this happened:

# The Mac physically detects a connection
kernel: (IOAccessoryManager) IOPort::addTransport(): [Port-USB-C@2] Adding transport... (type: DisplayPort)
kernel: (IOAccessoryManager) ... driverStatus: 2 [Ready]

# The monitor signals it is ready (Hot Plug Detect)
kernel: (IOAccessoryManager) IOPortTransportStateDisplayPort::setHPDState(): HPD state changed! (hpdState: 2)

# After exactly 7.5 seconds, the Mac gives up
kernel: (IOAccessoryManager) IOPortTransportStateDisplayPort::setActive(): [Port-USB-C@2: DisplayPort] active: NO

The Diagnosis: The Mac „sees“ the monitor and sets it to Ready. However, it fails to complete the Link Training (the negotiation of video bandwidth) within the critical 7-second window. Consequently, macOS sets the port to active: NO to prevent errors.

Troubleshooting: What I Tried

Before finding the solution, I went through the usual suspects:

1. Monitor Hard Reset: Unplugged the power, held the power button for 15 seconds to drain the capacitors.
2. Clearing Display Configurations
3. OSD Settings: Checked for MST (Multi-Stream Transport) and USB-C Prioritization settings directly in the monitor menu which I could not find

The Solution: The Cable Paradox

Even though the original USB-C cable had worked flawlessly for months and years and continued to pass USB data (mouse/keyboard) after the update, it was the root of the problem.

Why? Driving a 4K signal at 60Hz requires extremely high signal integrity. The macOS update likely tightened the tolerance levels for Link Training. The old cable no longer seems met the 100% requirement for video data, even though it was „good enough“ for simple USB data transfer.

The „Success“ Log (After Replacing the Cable)

With a new, high-quality thunderbolt 4 cable, the terminal output changed immediately:

# The breakthrough: Status stays on YES
kernel: (IOAccessoryManager) IOPortTransportStateDisplayPort::setActive(): [Port-USB-C@2: DisplayPort] active: YES
kernel: (IOAccessoryManager) IOPortTransportStateDisplayPort::setActive_block_invoke(): [Port-USB-C@2: DisplayPort] active: 0 [NO] -> 1 [YES]

# Hardware confirms the Link-Sync
kernel: (AppleTypeCPhy) AppleTypeCPhyDisplayPortInterface::setLinkRate: ... completed with 0x00000000

# Audio services load (since video is now stable)
audiomxd: Core audio route changed to [DisplayPort, 10AAAAA1-...]

Conclusion

If your USB-C monitor goes black after a macOS update but the USB hub keeps working: Don’t trust the cable! macOS is incredibly strict during the DisplayPort Alt-Mode handshake. Swapping the cable for a Thunderbolt 4 or certified USB-C video cable is often the fastest fix, even if the old cable „worked fine yesterday.“