External USB-C Monitor stopped working in macOS Sonoma

I had the same problem with my M2 Pro and LG HDR 4K after upgrading to Sonoma 14.2. Spoke to LG Support. They had me do a hard reset of the monitor - unplug the monitor, hold down the power button on monitor for 10 sec while unplugged, then plug monitor back in...and it worked!

2018 Mac mini i7. Lost ability to drive 2nd monitor after upgrade to Sonoma. Two identical Dell 24" monitors (with two identical USB-C to DisplayPort cables). I upgraded the cable of the 2nd monitor (non functioning) to a Cable Matters "32.4Gbps / Thunderbolt 4 / USB4 / 8K" certified cable from Amazon. This resolved the issue.

Try it in Safe Mode.

Safe Mode does a number of different things. On older Intel Macs, hold shift at startup, but have your userid and password at the ready.

A parade of unusual things happens.

• Your Mac loads just enough of the kernel to do a disk check. Then it proceeds to do a disk check. This can take an extra about five minutes.

• your userid and password are required, even if you normally auto-login. So have them handy.

• Your Mac adds ONLY a minimal set of Apple-Only extensions, Not including graphics acceleration extensions. Screen updates will therefore be wonky and slow, but it ultimately should be correct.

• Your Mac assumes defaults for as many settings as possible, including screen resolution. This is the key for re-setting the screen, but there is a little more to it: Resolution is likely to be lower and settings ordinary. Use this as a starting point to customize settings to your liking.

Any changes you make in Safe Mode will "stick" in regular mode after you restart.

• after restart in normal mode, your Mac will take slightly longer to start up [once] because it rebuilds some system caches.

"Works in Safe mode, fails in regular mode" implies "It's something you added".

How to use Safe Mode on your Mac

Use safe mode on your Mac - Apple Support

The problem was at the KVM switch. The vendor provided already a firmware update and now everything is working again. The cable lenght was not the issue in this case. Nevertheless, using a quality cable is always a good idea to exclude this part already from the beginning as a potential source of failure.

I fixed it with a better a cable. After Upgrading to Insomnia, the monitor behind the Switch went dark. The not-working-cable was the one that was delivered with the box (I guess) from I-tec. But I still had a spare usb-c cable of high standard (I bought it a year ago for some other use), and now it works. It is 0.8m USB 3.2 Gen1. From Logilink. Best, Michael.

Same thing for me - just stopped working after a reboot and presumptive update to Sonoma 14.5 - one of two LG HDR 4K monitors connected w/ USB-C cables provided by LG (provided in the monitor box) directly into a MBP M2Max USB-C port (one on each side). both cables under 1m (or whatever ridiculous limitation that was mentioned). I had to unplug monitor power and hold down the monitor power button for 10 seconds and replug before monitor would render. Unplugging power and usb-c cable at both ends did not work only the power interruption and 10 sec press of power button on LG monitor fixed the issue. For reference sake, these monitors have worked w/ almost no interruption for 2-3 years w/ another intel MBP as well as this machine for over a year and no issues whatsoever beyond having to unplug and replug the usb-c cable occasionally (until now).

<<. I've been using the cable that came with the monitor since I got it and it has been working perfectly ... >>

cables that shipped in the box are infamous for being just good enough to keep you from taking the device back to the store. if yours is not CERTIFIED, you need a cable that is CERTIFIED and has those markings, and is appropriately SHORT.

<<. ...before upgrading to macOS Sonoma... >>

Yes, but you changed something. Sonoma is not the same as the previous MacOS. Each successive version of recent MacOS has tightened the tolerances for what is acceptable for errors generated in data transmission to the display. Get a known-good CERTIFIED cable and that may solve your problems.

I have a Dell monitor - working great and then was not recognized one day.

Followed a suggestion here and removed the power completely from the monitor for 15 seconds and plugged it back in. Then attached the Mac - monitor is recognized again.

I have a similar LG ultrawide and the same issue. Try a hard reset on the monitor. Unplug monitor, hold power button for 10 sec. Reverse the cable. If not, HDMI->usbc will definitely work. Usbc port on the monitor doesn’t work reliably anymore. Something to do with the dual direction with power. My battery no longer charges on that port and after the upgrade the battery went bad. Terrible experience all around.

scaledoctor wrote:

<< using my Aten CS1953 KVM switch using USB-C.>>

the total length of the USB cable for a KVM switch that does NOT have active drivers in it is still one meter, maximum.

If and ONLY IF your KVM switch has active drivers on the outputs, (such as Docks or switches that convert to a different protocol or extract DisplayPort from USB-C) then each leg (in-leg and out-leg) is limited to one meter, maximum for USB-C, and similar lengths for DisplayPort at high resolutions like 4K.

What I read into that is, "don't always assume that the Mac is the problem -- it is also possible your display is confused, and in a funny state."

In the case of the Apple Studio display, that is frequently the case. When the Studio Display misbehaves, it needs a timeout. Disconnect all power and cables and let it sit by itself for a quarter hour. (Facing it into the corner or moving it to the 'naughty stair' is optional.)

I'm having the same issue with my Phillips 32 inch widescreen monitor. Everything was working perfectly with my monitor. I upgraded to Sonoma and the monitor did not work. I was using a TRIPP-LITE adapter connected through HDMI. Connection through the TRIPP-LITE does not work through HDMI nor USB-C. When I plug my computer into the AC adapter and use the USB-C adapter, it works. If I try to run my monitor off battery, it seems Sonoma doesn't have enough power to display both monitors. This means I cannot use USB 2.0 adapters (as all thunderbolt/usb-C ports are being used) with two displays. Apple - please fix this. Unplugging my monitor to power my external hard drive is a joke!

<< My current USB-C cable from the MacBook Air is around 2 m length. Do you think a shorter one could be the solution? >>

Yes, exactly that.

later versions of MacOS are more demanding in seeing error-free trasmisson.They ask the display if any transmission errors have occurred, and if they have, the display goes dark or its resolution is reduced.

a proper USB cable to support a high resolution display on recent macOS is ONE METER in length or shorter. And it has the USB-C SuperSpeed PLUS icons on both ends or on a tag:

uncertified, unmarked cables are very often less than excellent quality. Cables 'shipped in the box' are often "lowest bidder" cables, and are just good enough to keep you from returning the display on the first day.

The FRONT USB-C ports on Mac Studio MAX (not Ultra) are NOT fast enough to run a high resolution display. They are explicitly limited to 10 G bits/sec maximum, with no ability to 'turn around' the inbound data paths for momentary outbound use to attain the required about 20 M bits/sec required..