After Monterey 12.3 upgrade, can't run 2 usb-c monitors

Indeed I've seen it with LGs monitors on 3 different Mac mini 2018, all using USB-C connections and one of the displays "vanished" after upgrading to 12.3.

My take: Macos cannot distinguish the monitors, because LG monitors (at least those that I have 32UL750) Don't present a serial number and thus 12.3 is only "recognizing"one of them (the one that "answers" first).

Macos up to 12.2 had a terrible time properly identifying the monitors and they would often swap on boot or sleep (I have my own keyboard shortcut to force them to swap). Again because there is no way to do it otherwise.

If they have "fixed" this swap issue, there could be an explanation for the whole problem, if monitors are indeed "equal".

We have 3 minis back in the game by switching one of the monitors to HDMI and used the following procedure to have it recognize the monitors properly:

• Shutdown
• Leave the HDMI connected and remove the USB-C cable from the mini (not the monitor as the cable may be active)
• Start, check the monitor, screen size and refresh rate. Shutdown
• Now power the second monitor and connect the USB-C cable
• Start the machine.

On all 3 minis both monitors came back.

I will try the procedure with 2 USB-C later, but for now I have to get back to work.

The 2018 does support dual displays staggered on the TB/USB-C ports.

Like you, I've had dual Samsung monitors connected to the TB/USB-C ports of my 2018 Mac Mini for years.

From the number of 12.3 update issues on the board, something is not right and we need to let Apple know.

Feedback - Mac mini - Apple

Feedback - macOS - Apple

hcsitas, citation please? You've asserted that twice now, but the specs say nothing about an HDMI being mandatory, unless using three displays.

medium-cool, +1 for filing a bug report. If you have a time-machine backup you could also try reverting back to before the update.

They don't read threads on this forum, and the Apple Store will not be able to fix anything. Only workaround at the moment is to boot w/ 1 screen on the native HDMI port, then plug the 2nd one via USB-C once booted.

The issue has been lurking for months, I can find posts dating back to January from people who tried the beta version on their mini. It's obvious the relevant folks at Apple are unaware. Best is to open a bug report, like many of us did => Feedback - macOS - Apple

The more the merrier, only way they might eventually notice there's a problem.

I believe the 12.3.1 release also seems to have fixed my issue. My HDMI monitor has stayed working across multiple reboots and monitor power cycles after updating to 12.3.1. I will verify the monitor still works properly in the morning (I normally turn the HDMI monitor off overnight).

I still find it odd that my experience seemed the opposite of most others' (my eGPU output through multiple adaptors was working fine, but built-in HDMI was failing on 12.3), but I'm happy Apple put out a dot release fixing this (and apparently multiple other bugs) relatively quickly.

Some good and some bad news...

A friend that has 2 minis 2018, both with dual displays, reported to me today that his setup (HDMI+USB-C) stopped working when he powered on his machine (cold boot). This time the HDMI display was not working. Mine was on sleep and returned ok. I attempted shutdown/boot and the same happened to me, but with one mild different: the icon arrangement was not showing the icons that were supposed to be on the other (blank) display, as if the other monitor was there. I tried powering the HDMI off and on and the icon arrangement changed, showing all icons now (overlaying from both displays).

The good side: my take is that it's not a hole that was plugged, but rather some bad bug indeed. If we need to see this fixed, we *all* must file bug reports to make them see the issue. The Mini 2018 is still being sold by Apple, that give us hope.

Also, for the shutdown/boot, once it's working, if you do the following it's much simpler to get it back into the game with a single pass:

• Before powering the mini, turn the USB-C monitor off (monitor power switch will do it (see bellow)
• Turn your mini on, and let it boot solely on the HDMI monitor
• Turn the USB-C monitor on

We did this a few times and it worked. It's not ideal, but allows one to consistently get a working system on every boot... or at least until we find yet another "detail".

As for powering the display off, if your monitor uses a mechanical switch, I recommend adding an extension cord with a power switch (many cheap options on Amazon). You don't want excessive wear on the monitor switch.

Hoo-boy. An update: I have the (same) 12.3 installation working now with both monitors (eGPU + HDMI), but I'm not sure exactly what I did got it working again. Here are the steps I performed, to the best of my memory:

1. Intending to set my Mojave partition as the startup disk, I noticed it was absent from Startup Disks in 12.3's System Preferences (perhaps because that volume is on an HFS+ Journaled USB3 external drive?). So to reboot into Mojave, I had to hold down the Option key on startup.
2. When Mojave booted up again (after a double-boot sequence), the Apple logo actually started on the main/eGPU screen, with the HDMI output blank. At the end of the boot sequence, the eGPU screen went blank, but the HDMI screen still didn't show up.
3. I then forced shutdown by holding the power button. Before restarting, I pulled the eGPU thunderbolt 3 cable, leaving only the HDMI connection plugged into the Mini, and booted up again. I actually meant to boot into Mojave again, but forgot to hold the Option key down this time, so it actually booted into 12.3 (since that was still the "blessed" system). To my surprise, this time the boot and login screen showed up on the HDMI monitor, and completed bootup successfully!
4. I then plugged in my eGPU cable. Now, both screens were visible, though reversed in orientation and arrangement to what I wanted, but I was able to reconfigure things without issue.
5. I then retested with another reboot into 12.3, this time leaving both screens connected. This boot sequence went as I'd expected before (and what I saw during my "test run" last night): initial Apple logo appeared on the HDMI screen, but then both monitors displayed, with the login on the main/eGPU monitor I'd last specified. After logging in, both are now working as expected.

Hopefully this configuration sticks (I always set my Energy Saver prefs to never sleep the displays or the machine). If I run into issues again, I'll try to remember to report back here. I realize this thread was started with people having issues getting *non*-HDMI displays to work, but my ordeal seems to point to something just being generally wonky in 12.3's display detection. If people have an older backup they can reboot into, it might be worth trying the steps I performed to get this working.

This is definitely an issue, Apple shall release an update to fix it because it's real annoyance.

My configuration is one 4K through HDMI and one 4K via USB-C, the HDMI one failed to be turned on after each restart. And my tested workaround:

1. Full shutdown to prevent electricity shock, then disconnect the USB-C cable from the port on the mac mini connecting to the good one.
2. Turn on the mac mini, now it drives the HDMI monitor to work.
3. Leave the login screen on, re-connect the USB-C cable.
4. Now both monitors work.

The point is to let the machine drive the "bad" monitor to work first, then light up the others.

And disconnecting the power cord doesn't work because mac os somehow still can recognize the monitor if the data cable is still connected on.

You’re welcome! You’re on dual monitor per spec except for shutdown & restart. That’s livable with since neither are required for routine use, including overnights. In the meanwhile, disable Sleep and let the monitors auto-shut on timer as shown below, who knows it might even improve the shut/restart issue.

I've seem reports with dual Samsung too, so it's not limited to LG monitors. My guess it's the way Apple decided to distinguish the monitors does not take into account the fact that whatever they are using don't account for "homonyns" (for a lack of better word here). When I was playing with the display swap problem I noticed that all the internal identifiers for both monitors were 100% equal thus impossible to distinguish. For a while I've been using my keyboard shortcut to swap the monitors whenever I reboot or wake my Mac mini and the displays were reversed.

Let the monitor manufacturers fix it? Hard to believe they will do it. How do you expect LG to fix this:

The SERIAL is "BLANK" (looks like a cheap solution). There is no way to provide a patch that will install a serial or other mean of identifying the monitor. However, it's pretty easy for Apple to synthesize a serial or identifier based on the port/path of the connection. Just don't do it the way Windows do it, as it create yet a ton of other problems.

The sad note is that I've seen similar problems with Linux: the device in the class that "replies" first gets the role. Happens a lot with network cards. So a OS only solution may be complex too.

At least we can have a working configuration, albeit not perfect.

Per the spec, one display must be connected over HDMI for a dual-monitor configuration. I understand your 2 USB-C combo worked in the past but that’s no guarantee it’ll work in the future, especially if the spec doesn’t allow it. Try it with one over HDMI and the other USB-C? If it works, you can file a report with Apple, maybe they’ll fallback to what worked before.

The 2018 Mac mini has four Thunderbolt/USB-C ports on two Thunderbolt busses. Make sure the displays are not using the same bus, i.e. on the two left or two right connectors. Plug one display into one of he left two connectors, and the other into one of the two right connectors.

If they are the 4K LG displays that Apple used to sell, they come with both USB-C cables and Thunderbolt 3 cables. The Thunderbolt cables should have lightning bolt icons on the plugs. Try the Thunderbolt cables instead of USB-C.

Also, if they are the LG 4K Thunderbolt displays, they will have two Thunderbolt ports each, so you could try daisy-chaining them from one computer port using Thunderbolt czbles.

You have two displays, the spec mandates one on HDMI. That said, it could be a bug that’ll get fixed in the next release or a long-standing bug that’s been plugged in the current one. Both are possible and have happened in the past. Your best option is to file a report. Before that, see if strictly following the spec still works. Good luck!

I have this issue too and thanks for suggestions.

I got both monitors to work temporarily by connecting both HDMI and USB-C/DP into one monitor and USB-C/DP into the other. However, after shutdown and startup the dual doesn't come back unless I start messing around hot unplugging and plugging.

I've tried booting with one HDMI and one USB-C/DP as well as booting with each monitor connected to USB-C/DP on different buses - nothing.

Looks like I'm back to single monitor until a fix as I don't want to mess around with the cables every time I fire up!