External Monitor not Detected after upgrade to 12.3

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.

Some displays include their own private "sleep" settings for the display alone. This can allow the display to enter its own sleep mode, on top of the Mac's not sending it data. A display that is sleeping on its own cannot respond to the Mac's query, and will stay dark.

I did have the same issue with a LG 32 4k Monitor. I unplugged the monitor from power for a while. Plugged it back-in to the MBP and re-powered the monitor.

After that the monitor was recognised by the MBP M1Pro again and is working fine since. Strange, but true ;-)

Another user in another forum pointed this out to me and it... worked?

Disconnect your monitor

Unplug your monitor

Wait a minute or two

Plug your monitor back in

Connect your monitor

This... seems to have worked for me after trying all the more complex solutions (safe mode etc.)

I may have found another workaround that is almost as frustrating as the original bug but at least has me working on my second display currently:

I have an Apple TV.

Airplay to Apple TV, instead of hdmi out to tv, hdmi out to monitor

Unplug from Apple TV, connect to Computer, wired connection, full resolution (same as before 12.3)

Hope that helps anyone in that niche situation.

There is at least one other "question" about this issue, which prevents my mac mini from waking from sleep while connected to a USB-C display that itself has been put to sleep when the box slept. This appeared right after the 12.3 update. Both questions have Apple-recommended answers that don't fix the issue for most people. I wish there were a way to submit an old-fashioned detail-laden bug report without some kind of deep learning parser involved... When I say "won't wake up" it just tells me to go the Apple store. For now, I'll disable sleep in my display.

I have an iMac (Retina 5K, 27-inch, 2020) and I recently purchased a portable monitor Lepow Z1 USB-C and it was working properly with the previous version of macOS. As soon as I upgraded to 12.3, the monitor could not be detected and it shows the "No signal" error. I did all the jobs that were found on the Internet and Apple website like unplugging and plugging again Clearing NVRAM, connecting on the time of turning on, logging in a safe mode, changing the USB-C cable, using the Mac Original Thunderbolt 3 cable and so many other unusual works that somebody said like changing the direction of the cable and so, but no change. I already talked 2 times with Mac support and they still could not fix the problem. The same monitor with the same cable is working with my MacBook which is under the previous macOS version.

It works with some of the USB-C to HDMI adapters but not all. It works with a cheap one which is a no-name random USB-C HDMI adapter but I have another that I purchased more expensive but doesn't work with that.

I just recommend downgrading if you just upgrade it till the moment that a real solution will be there.

I have a similar issue. I have an external LG monitor connected to my iMac; ever since I upgraded to OS 12.3.1. my iMac would not wake up from sleep or even start-up after I shut it down. It gets stuck in a restart loop. I have talked to Apple technicians three times, and they walked me through the PRAM/NVRAM reset, SMC reset, as well as reinstalling the Mac OS; none of this has fixed the problem.

Eventually after going through the forums, I suspected that the problem maybe that OS 12.3.1. has issues with external monitors. So at this time, my work around has been to disconnect the external monitor before start-up; I have also been disconnecting the external monitor every time I walk away from my computer (i.e. before it goes to sleep). I am only connecting the external monitor to the iMac once I have logged in; this is cumbersome, but now I am able to work on my iMac.

I am hoping that apple will fix this problem soon! These are expensive computers and it is ludicrous, that we keep running into issues with each OS upgrade.

I have the same issue with my LG 4k Monitor connected via USB-C to my MBP M1Pro.

Sad Mac :-(

I take this back. I'm back to my external display no longer working and the steps I put in last night no longer work to "activate" my external display. Thanks Apple!

Be sure that Apple support has heard form you and they file a trackable Bug Report on your behalf:

Get Support

.

same issue here. I'm going mad.