External USB-C Monitor stopped working in macOS Sonoma

a USB-C cable for a high-resolution display like that one requires a certified cable rated for USB 3.1 10 G Bits/sec or better with this logo or better:

NB>> . and length under one meter maximum.

For you guys reference of my case:

• connect to monitor using type-c cable
• connect to monitor using HDMI cable together
• if it's not auto switched on monitor with display, switch manually in monitor to HDMI mode.
• once it displayed correctly, unplug HDMI cable, and type-c signal detected

I got the same, m2 with sonoma 14.4.1 on msi monitor

1 usb-c port closest to power socket stops sending signal randomly,

yes, restart fixes that annoying issue

BUT I noticed that flipping usb-C plug over 180° and connecting again makes this port work again, yes I know its symmetrical, which even more stupid why it fixes the issue.

PS works with old macbook 61W USB-C power adapter, which I still use for all laptops that have usb-c port, having separate charger for each device is just too much.

If the display doesn't work at all, try putting the macbook in sleep mode, then wake it up... it solved the problem for me.

Had the same issue after updating to Sonoma from macbook air m2 - My monitor stopped working after that. Changed the HDMI cable with another one in my house then worked again... how odd!

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I made a small diagram of my setting which worked flawlessly before the update. The KVM switch (3 inputs: DP,DP,USB-C) has an own power supply, my monitor a resolution of 3440x1440. After the update the MacBook Air is not detecting/activating the external display for some reason anymore. My current USB-C cable from the MacBook Air is around 2 m length. Do you think a shorter one could be the solution?

if using the proper cable is not enough to solve your issue, be sure to talk to official Apple support:

Official Apple Support

Do NOT let them tell you it is fine if it is NOT fine.

if your issue remains unsolved after their best effort, Be sure to have them file a Bug Report.

<<. Now I have to restrict my space because the cable can only be 1m? >>

The data rate used by a high-end, Hi-DPI, hi-color display is THE most intensive data stream produced in your Mac. It is Faster and more unrelenting that the data read from disk. The data rates are approaching the physical limits of switching speeds of electronic devices, so cable-lengths are limited.

if you reduce those requirements, such as by reducing the resolution of your display, you can use a longer cable.

LG Ergo 32un880 appears to be a "4K" display with

USB-C and

HDMI inputs.

HDMI is bandwidth-limited and trouble-prone at high resolutions.

to use an HDMI cable at that resolution on a modern Mac requires:

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"Premium High Speed HDMI cable" or that + "with Ethernet" --OR--

"Ultra High Speed HDMI cable" or that + "48G"

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

Using external drives that do not have their own power supplies is BEGGING for trouble. If the computer sleeps. MANY different models of drive will spontaneously eject, and MacOs will throw error "disk not ejected properly".

I have the same problem. After upgrading to Sonoma, my external LG screen (34WN80C-B) is not detected anymore when using my Aten CS1953 KVM switch using USB-C.

Any ideas? I contacted the Apple support in the meantime...