macOS Sequoia 15.3 wont let me detect my second display

<<I have a solution (I'm using 15.2 but should work in 15.3 too). After spending 40 minutes on the phone with apple, we figured out how to solve this. In the Finder, go to the "Go" menu, select "Go to folder". Type in "~/Library/Preferences" There is a preference file with the name that starts with "com.apple.windowserver.displays." then has a random long number, then ends in ".plist". Right click on this file, and "Move to Trash". Restart your computer. Done >>

"the solution for me was very simple and strange"

Mac Mini M4 Pro. MacOS 15.5. 43" LG C4 TV x2 used as dual monitors. The LG TV used to extend the display started intermittently not being recognized after updating to 15.4.1, then was never recognized after updating to 15.5.

I tried everything after scouring the Internet without even temporary success. Ultimately I was limping along using the second monitor via ApplePlay at far less than its 4k resolution as the Mac Mini through Thunderbolt 5 port through USB C to HDMI adapter ("Cable Matters 48Gbps USB C to HDMI 2.1 Adapter for macOS, Supports 8K@60Hz / 4K@240Hz HDR, Thunderbolt 4 to HDMI 2.1 Adapter, Compatible with Thunderbolt 5 Laptops") through 10k HDMI 2.1 cable (10K 8K HDMI Cable 5-Pack 6.6FT, HDMI 2.1 Cable Bulk Ultra High Speed 48Gbps 8K@60Hz, 4K@120Hz Braided HDMI Cord eARC, HDR10, HDCP2.2&2.3) to the LG C4 TV.

I pulled the HDMI cable, flipped it, and then it worked again.

For about 1 minute, then the 2nd TV being used to extend the display was again not recognized by the MacMini.

I then pulled the HDMI cable only from the TV for a few seconds and after plugging the cable back into the TV the Mac Mini again recognize the 2nd TV and appropriately extended the display. (Previously detaching and reattaching the cable from the Mac Mini, switching ports on the Mac Mini, detaching and reattaching the adapter from the HDMI cable were without benefit.)

(Still working after 2 hours, so keeping fingers crossed!)

This has nothing to do with the displays, monitors,... in my opinion. I have been using the same setup with my old MacBook Pro and new MacBook Pro for about 5 years. Two external monitors - one display port / one HDMI. I have NEVER had an issue with the external displays coming online until upgrading to 15.3. It is always the HDMI display that does not come back after the system sleeps.

I have found that power cycling either the monitor (easier of course) or the laptop will bring all three back online and they will all remain fully functional until the next time it sleeps.

I wrestled with this problem after upgrading to 15.3.1. Was about to use Apple Support to erase 15.3.1 and go back to an older version when I stumbled on a fix after reading all the comments in this thread. The display monitor is black - but you can see it in system settings/displays. When I change the refresh rate from 75Hz to 60Hz and then wait about 5-10 seconds, the monitor comes to life. It's definitely a lower quality display, but at least it's working now. Here's a picture of what worked for me. LMK if it helps to solve your problem! Also, Apple still needs to solve this software issue as we need our external monitors to be the high definition we purchased!!

[Edited by Moderator]

Deleting the following two files and restarting resolved the problem for me (Sequoia 15.4.1):

1. Open "Terminal"
2. rm ~/Library/Preferences/ByHost/com.apple.windowserver.displays.*.plist
3. sudo rm /Library/Preferences/com.apple.windowserver.displays.plist

Nothing worked except this: I disconnected the main monitor, restarted the computer with the second monitor. Once it detected the second monitor, I connected the main one. It worked until I restarted the computer.

THIS WORKED!! read below:

Forget about NRam reset, Hz settings, new cable, hub or disconnect everything:

I just created a guest account and external monitor showed up. Then I logged out and back to my main account and the monitor is still there.

Sometimes it works to simply use a different Thunderbolt Port, but I had to use a completely different Thunderbolt to HDMI adapter to get the Mac to recognize the external monitor.

Once it recognized the monitor on the new adapter, I was able to switch back to the old adapter and it continued to work.

I had the same problem and thought my second Apple 27" Tb screen connected to a M2 Mac mini had died, but just to check, I connected it to another M1 Mac (MBA) and it came good instantly and then reconnecting to the M2 MM worked too.

YMMV but it seems to me that somehow Sequoia can disable a second Tb screen which then requires a reset on a different Mac. This is the third time it's happened to my dual Tb screens and a "reset" on another Mac has worked every time.

RetiredLegalBeagle wrote:

I have the same issue. My desktop is a 2021 IMac M1. When operating Sonoma, I had two external Dell monitors working, in addition to the main display, through a DisplayLink Plug and Display Hub. One monitor worked with a thunderbolt to HDMI cable and one worked through a HDMI to HDMI cable. No problems. Worked like a charm. Download Sequoia yesterday and monitor on HDMI to HDMI is black. "No HDMI signal...." Called Apple; spoke to less than sympathetic tech who said my problem is with the DisplayLink folks. "But," I said, "there must be something in your new OS since it worked fine in your old OS. She said, "I'm sorry."

The iMac (24-inch, M1, 2021) supports a maximum of one external display.

iMac (24-inch, M1, 2021) - Technical Specifications - Apple Support

You connected two – at least one of which you connected in an unsupported fashion, using a workaround.

One drawback of using a workaround is that, if there is any change that breaks it, you must rely upon the third parties who supplied you with the workaround to fix things. Apple never promised that you could attach more displays than the Technical Specifications say that you can.

My guess would be that if you contact the company that manufactured the "DisplayLink Plug and Display Hub", they will point you at Synaptics. Whether Synaptics has new versions of their drivers that fix the problem yet, I don't know. But that's the site you will probably need to keep monitoring.

Synaptics – DisplayLink Graphics

DisplayLink Support – Troubleshooting: macOS

I was able to get my second monitor working again by unplugging it, then deleting the windowserver.displays.#### plist.

Then I rebooted, waited for login prompt. Plugged the second display in at that point, then it came up.

Thank you. It was easy to just trash this file and restart and problem solved.

apple.windowserver.displays.<hex_string>.plist .

That worked. I found this file and deleted it and restarted my computer: The file path is /Library/Preferences/com.apple.windowserver.displays.plist.

here's the solution. close the mac. disconnect ALL cables (all!). close the monitor or just disconnect ALL its cables. wait 5 minutes. reconnect and boot all devices. worked for me and I had the exact same problem.

This fixed it for me.

Delete ~/Library/Preferences/ByHost/apple.windowserver.displays.<hex_string>.plist and reboot.