External Monitor Issue After macOS Sequoia 15.1.1 Update

Hi all,

Last week I updated macOS to version 15.6 (24G84) and it seems that Apple has finally solved the issue with my monitor (Dell P2721Q).

I’ve been using it every day and the monitor hasn’t flickered yet.

It’s quite frustrating that it took more than eight months to get this issue resolved, and there’s still no RCA or any explanation for the problem, even though I had an open ticket with them through AppleCare+. Their last message to me, over two months ago, was that they didn’t know what was going on and that I would have to wait.

During this time, I tried multiple versions (including unstable ones), sent the machine to Apple for more than a week, and spent a couple of dollars on new cables and adapters.

I really hope that the issue does not appear again in a future release.

Ok, I found a fix for me. It's all backward.

I had a "higher quality" / faster HDMI cable that I have been using fine for the last year with lower OS versions. During the general troubleshooting with Apple support, I remember I still had the old "Amazon basics" cable that I had used prior to the better-quality cable.

I swapped the old cable, and now it works fine.

"Newer Cable"

https://www.amazon.com/gp/product/B0CJLC72MG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

"Older Amazon Basic Cable"

https://www.amazon.com/gp/product/B083KX4F6K/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

An obvious culprit is that the old Amazon cable explicitly states the 60hz refresh rate while the newer cable lists higher refresh rates.

Now this has worked for a year so I think this suggests that there is something in the OS update that is forcing display connections closed if there is some sort of dissonance between the system refresh, connection, and display refresh.

At the end of the day I'd rather use my higher quality cable that has been good for the last year, but who knows when Apple will figure out this bug they introduced.

Exact same issue for me as OP. After updating to Sequoia 15.2 (I don't recall what the previous version was), the flickering issue occurred immediately after the update. Nothing else has changed about my set up after the update, so I feel very confident that the 15.2 update is the root of the issue.

Here's my set up prior to Sequoia 15.2 Update:

• Macbook M1 Max, 16-inch, 2021
• MacOS Sequoia 15 (not sure which previous version)
• External displays:
  • Working normally: "Display 1", LG 27GN650-B (connected via HDMI port)
  • Malfunctioning, flickering: "Display 2", LG 34UM69G-B (connected via usb-c port, USB-C to USB-C display cable)

Steps that failed to resolve issue:

• Follow steps 1-4 outlined here. Did not resolve issue.
  • Unplug cable and monitor power cord
  • Mac is already at latest update
  • Restart Mac
  • Change color profile of external monitor to Color LCD
  • Change refresh rate to 60 hz
  • Disabled true tone/night mode where applicable
• Changed USB-C cables, and tried plugging USB-c cable into different USB-c ports. Neither resolved issue
• Swapped display cable from USB-c/USB-c to USB-c/DisplayPort. Did not resolve issue.

Steps that resolved the issue so far as of Dec 19, 2024 at 11:06:

• Swap display cables between Display 1 (working) and Display 2 (malfunctioning):
  • Display 2 is now connected via HDMI cable to mac HDMI port
    • Color profile: Color LCD, 60 Hz refresh rate, no true tone/night
  • Connect Display 1 via DisplayPort to USB-C mac port

Since swapping display cables between external monitors, the flickering has stopped. I will update accordingly if anything changes.

Update Dec 19, 2024 2pm: Unfortunately, the flickering still occurs but it's less often and less intrusive. The flickering happens maybe once an hour. And, instead of my mac thinking the display has been disconnected and rearraning all my open windows, it's just a brief off/on flicker and no windows move.

After reading this thread I tried one more thing - replacing my $70 Mac USB-C to HDMI (Multiport) adapter with a much less expensive Anker USB-C to HDMI adapter and it fixed this problem. Something in Sequoia broke the mac version of this adapter, which is ironic considering it's so expensive. I also tested with a Nintendo Switch USB-C to HDMI adapter with the same results. The Anker and Nintendo adapters work fine, the disconnecting/blinking went away. Only occurs with the Mac brand adapter (MacBook Pro > Mac USB-C Multiport > HDMI cable > LG Ultrawide monitor).

Nope, didn't work for me.

"I have the same issue. The workaround is to power the monitor first with its own power supply and then connect the video (usbc or hdmi). Power and video delivery over 1 usbc cable seems broken."

Sequoia 15.2 External Monitor FIX Steps:

1. Delete /Library/Preferences/com.apple.windowserver.displays.plist file
2. Empty trash bin
3. Shut down MacBook (not a restart)

I use a LG Ultrawide as a second monitor. Works perfectly fine again now. Good luck! My Applr Rep had no idea until I found this from a Reddit user.

thanks for this, the usb c port swapped work for me MacBook Pro M1, os15.2 , as far as I can tell the one port thinks the monitor is 1080 max but the other port thinks it is 4k. I am using a cheap dock to do this so it is HDMI into the dock and the dock is usb into the Mac itself. swapping the dock between the ports on the Mac is what fixed it.

RESOLVED (for me, for now)

I've been watching this thread because I have had the same problem everyone else has. Here is my setup:

MBP M1Max

3 identical LG 1920x1080 monitors (27MN60T)

Main display (center) uses HDMI to HDMI

Extended displays use HDMI to USB-C

Refresh Rate: 75hz (on all three)

Color Profile: LG FULL HD

No docking station.

Because MacOS can get confused on waking and swap my 2 extended displays (its own frustrating bug that has been present for at least the last 2 operating systems), I HAD been using VGA to USB-C by way of an adaptor for 1 of the 2 extended displays. That is the screen that was blacking every couple minutes once I updated to 15.1. As I was trouble shooting I noticed that screen was fixed at 60hz refresh with no option to adjust to the 75 the other 2 used. I bought an Anker HDMI to USB-C cable assuming that as long as I was replacing a cable I would try a decent quality one rather than the least expensive.

I have been running now since Saturday with no screen black out. I have no idea if this will help others or not, but thought I would share, just in case it might.

I had the same issue. It recognized one of my external monitors connected via HDMI, but not the one connected via DisplayPort. I tried swapping cables, went so far to buy a new docking station that had HDMI only connections. Nothing worked. Tried deleting a plist that was suggested. I found a reddit post that linked back to another discussion and they mentioned unplugging the power cord from the monitor, leaving everything else connected. I just did that and my 2nd external monitor is working again!!!! Hopefully it helps others. This is the post that led me to it: https://www.reddit.com/r/MacOS/comments/1hc8mcz/comment/m22ql12/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

Same issue on Sequoia 15.3.1, using 4 different USB-C -> HDMI hubs or cables.

My monitor was not detected anymore after macOs upgrade from Sonoma. This monitor works on a PC with same USB-C devices.

After reading all the comments and reports, I wiped my mac and reinstalled Sonoma 14.7.4 (latest to date). Issue vanished.

What a shame, Apple.

I won't upgrade until I read a public statement from Apple saying the bug is fixed.

Hey guys,

I started to experience the same issue when updating my MacBook Pro to Sequoia 15.6. I've since resolved this issue for myself. I don't work for Apple in any way, so please don't get super mad at me if any of these things don't end up working for you...

Unplug EVERYTHING from your device, to include the charging cable.

I deleted these files:

• • /Library/Preferences/com.apple.windowserver.*
  • /Users/<USERNAME>/Library/Preferences/ByHost/com.apple.windowserver.*
  • /Library/Preferences/com.apple.dock.*
  • /Users/<USERNAME>/Library/Preferences/ByHost/com.apple.dock.*

I think there might be a chance that other folks docks might have a different "com.apple." file associated with it. I don't know what those would be...

When restarting the device, I reset the NVRAM. I have an Intel processor, so to do that, when restarting, I have to hold down the Option + Command + P + R keys.

See this link for more info on that: Reset NVRAM on your Mac - Apple Support

I let the device completely boot up and let it sit for a few minutes. Then plugged in the dock, then plugged in the HDMI cable. Works fine now... I've read on some of these forums that the issue reoccurs over time. Fingers crossed that doesn't happens.

Hope this helps. Cheers!

I have been using a DELL 4K via my Sony STR AV amp, and a Ultrawide directly from a Sonnet hub on my Mac mini M2 running Sequoia. Previous to the update, everything working perfectly for years. After the update, absolutely nothing would make my Dell work ... until I tried the solution offered by users here - and I AM SO HAPPY TO SAY IT WORKS!

Go into Library - Preferences - renaming the file to "com.apple.windowserver.displays.plist" to "_com.apple.windowserver.displays.plist" After you rename it, restart your mac, and your mac should detect your display now.

I had a similar issue this morning after updating to Sequoia 15.7.1. The external display was not recognized anymore after starting the Macbook M4. With my other Macbook everything worked fine, so I could almost exclude any hardware and cable issues.

What solved the problem was to go into the last Time Machine Backup and copy the file "com.apple.windowserver.displays.plist" from Time Machine backup in ~/Library/Prefences and paste it into ~/Library/Prefences on the Mac HD.

After re starting the MacBook the external display was recognized and is working fine again.

15.2. is available !

I just installed it on my MacMini and MacBook Air, returned to my previous adapter: the bug has been fixed obviously!

Hi again,

Updated to 15.2 and seems that the problem is solved now. Connected with Apple HDMI Adaptar in one USB-C and Charger in another USB-C.

Regards.