External Monitor Issue After macOS Sequoia 15.1.1 Update

Had the same problem with a Samsung Smart Display M7. Someone here suggested swapping the USB-C cable for HDMI. I did that and it worked.

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 rearranging all my open windows, it's just a brief off/on flicker and no windows move.

I suffer the same issue after upgrading to 15.2 (most probably from 15.1, where everything works fine) on a MBP M1 2020, connected to USB C hub with power + external display via display port + USB stuffs.

What I have done so far to solve this issue in this order (actually I don't know which one as an effect):

• ensure the 'Prevent automatic sleeping on power when the display is off' setting is enabled
• configure the MBP display as main, and switch it back to the external display
• reset the Amphetamine App settings to default, then in settings disable the option 'allow system sleep when display is closed'
• switch external display refresh rate from 120 to 160: it makes the external display not working at all, even after a reboot
• disconnect the USB C cable (external display + power)
• shutdown
• reset (or try at least) SMC and NVRAM/PRAM
• reboot

Then it finally works again when the display is closed \o/

I suffer the same issue after upgrading to 15.2 (most probably from 15.1, where everything works fine) on a MBP M1 2020, connected to USB C hub with power + external display via display port + USB stuffs.

What I have done so far to solve this issue in this order (actually I don't know which one as an effect):

• ensure the 'Prevent automatic sleeping on power when the display is off' setting is enabled
• configure the MBP display as main, and switch it back to the external display
• reset the Amphetamine App settings to default, then in settings disable the option 'allow system sleep when display is closed'
• switch external display refresh rate from 120 to 160: it makes the external display not working at all, even after a reboot
• disconnect the USB C cable (external display + power)
• shutdown
• reset (or try at least) SMC and NVRAM/PRAM
• reboot

Then it finally works again when the display is closed \o/

Exact same problem but with a Viewsonic monitor connected with USB-C to mini display port connected directly to my USB-C port on Macbook Air M1 while in clamshell mode.

I started working with my mac laptop open although I only use the extenal monitor. It seems to alleviate the problem but does not completely resolve it.

Tried all the things named everywhere and still got the problem to this day.

I also tried with a HDMI cable connected to a USB-c hub and got the same problem.

This setup (mac in clamshell with external monitor connected to USB-C port worked flawlessly for a couple of years before this Sequoia update (15.1).

I run trivia at a few places, and last night, I got my M2 Air to connect to TWO external monitors (huge TVs, both Samsung) on an HDMI splitter. I fiddled with the settings for a bit, and what worked finally was dropping the refresh rate from 144Hz to 60Hz. I came home and tried the same M2 Air with a KYY external, but nothing (although the same small KYY monitor works on a Dell). I hooked all three of my desktop monitors, a Dell, a Samsung, and an Acer (one at a time) to the M2 Air, and ALL worked with no adjustments to the refresh rate. So I started thinking it was the cheap KYY, but others in this discussion cannot connect with high-end monitors (yet I could last night and today). Frustrating.

To add to all of this, my new M4 mini connects to all three desktop monitors (again, a Dell, a Samsung, and an Acer), but the Dell (and ONLY the Dell) will go black for 3-5 seconds then come back on at least 5 times in an 8-hour workday.

Is Apple going to address this time-wasting ****???

Hey,

Had this exact issue after updating to sequoia 15.5 on my M4 macbook; my external LG display went from always being recognized automatically through usb-c to not being recognized at all (not charging nor displaying anything). Tried deleting the .plist file and restarting my mac but that didn't work.

What did work for me was:

1. Using hdmi-hdmi rather than usbc-usbc. This worked but was obviously still super annoying since now I would need a power cord as well rather than charging from usb-c, and the hdmi cable is way bulkier. So I kept trying.
2. Unplugging power from my monitor and then plugging it back in. Still no clue how this helped my mac find it since clearly the update was what broke it, but I'll take it

A bit disappointed with the fact that an update could break things like this, had to spend more than an hour fixing it...

Not sure if this has been mentioned. You can try to go to Mac Settings -> Privacy & Security -> Allow accessories to connect. This may have an effect on monitors.

Update: Checked on my wife's Mac Air M2 and hers works fine and she is running Sequoia 15.1.1. She has a different and newer brand of USB-C dock but still just one of those small USB multi-port docks with a couple HDMI connections and a couple USB connections. Nothing too fancy or expensive. Mine is a bit older but, again, no issue on Windows laptop.

I ended up downgrading my Mac mini to Sonoma because of this. Tried Sequoia 15.1, same issue. As opposed to the Macbook, on the mini I've no alternative to using the external display, and the display going off while I type vigorously isn't exactly my idea of fun.

Reply to: External Monitor Issue After macOS Sequoi… - Apple Community

I now remembered what I did to fix the issue last time this happened after a MacOS update:

Power your monitor off and turn it on again.

Didn't work for me...

Well, I got my Warrky USB C to HDMI Cable and there are no problems anymore since three days.

Again 50Hz at a LG 34" wide screen monitor. The HDMI adapter plug ist a bit hot (45°C), but ok...

Thanks again to Teichfrosch

Dear all,

I just upgraded to Sequoia 15.5 and our old external display (a TV/Monitor) showed the same flickering error, which was mentioned by many of you. This is very disturbing and shame on Apple that they still not worked out a solution.

After many tries, a simple change solved (sort of) the issue: The old screen's default resolution was changed to 1080p and after that to 720p. These both worked, no flickering. If the resolution was changed back to the default value (1366x768 ok, i know, it is ancient...) it was flickering very badly. Strange.

Tried this, no luck.

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

arcyarcy wrote:

I now remembered what I did to fix the issue last time this happened after a MacOS update:

Power your monitor off and turn it on again.

Just tried that on the MacBook (the mini is already back on 14 and will stay there until Apple fixes this.) Powered off the monitor, removed cable (HDMI) from both sides, waited 5 minutes, then reconnected the cable on both ends, powered on the monitor and ... in less than 1 hour it flickered twice, the first time 2 minutes after I powered it on! I guess the MacBook is on its way to 14, too...