I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro.
MacBook Pro 14″, macOS 26.0
Just to help others Deep System Reset by abhiraj_chatterjee worked here too.
So the culprit was corrupted display preference files left behind by the macOS Tahoe upgrade. Not the hardware, not the cable, not the display switch. macOS had cached bad display settings and was silently rejecting the connection every time based on that stale data.
To recap the full journey for anyone who ever finds this:
Unplug monitor and run The Deep System Reset script posted by abhiraj_chatterjee:
bash
rm -f ~/Library/Preferences/ByHost/com.apple.windowserver.displays*.plist
sudo rm -f /Library/Preferences/com.apple.windowserver*.plist
sudo killall Dock
Then restart, plug back in after restart -> display works again fine.
✅ Thank you! The second solution you wrote up (Deep System Reset) worked perfectly.
I've replied and upvoted to flag to others that this works post macOS 26.1 upgrade.
MBP M1 - HDMI to USB-C cable (connected via UGREEN USBC converter with Ethernet cable port and 3xUSB 3.0 ports) - HD monitor (KTC)♥️
I have a MacBook Air M1 and a Mac Mini M4, both of which were connecting to my external LCD monitors wirelessly. Both worked flawlessly until I updated to 26.1. Now, neither display is visible to the Macs.
There are no, and never were, any cables involved in connecting.
Has Apple yet acknowledged they have a problem and WHEN they might have a solution, ready to roll out,
pajacobs--
To get the attention you deserve, PLEASE start a NEW discussion.
If you did not immediately find your solution by reading through the existing postings, you likely do not have "exactly the same problem" and need to explain from the beginning. This discussion is already way too crowded with different problems posted by different users with different hardware, different versions of MacOS, and different displays.
Starting a new discussion will allow Readers to think hard about your exact situation and address the exact circumstances you are facing.
https://discussions.apple.com/post/question
Thank you, abhiraj_chatterjee. This is a huge problem, and I was relieved to learn I was not alone. I have been trying to resolve this issue for three days--trying everything I could find (including deleting the ~/Library/Preferences/com.apple.windowserver.<UUID>.plist ). With just one monitor, I was working with "one eye closed." Upgrading my HDMI cables to HDMI 2.1 cables made both monitors finally come on, but only as a mirrors of one other. In Settings, I still only saw one monitor.
When I tried your DEEP SYSTEM RESET and restarted, I FINALLY have two monitors working independently of one another. Thanks a million, trillion!
It's M4 Pro. The monitor is a Samsung 34" (CH890), connected via a USB-C to USB-C cable, no adapter or dock.
The external monitor was detected and working fine before upgrading to macOS 26.
<< there are reports online that my monitor model can have issues with the USB-C port on the back. >>
what make&model display is that?
I don't understand your line of questioning. How does this information help resolve the problem - the user says that the monitor worked before MacOs
I am facing the issue with HDMI monitors as well.
Thanks for the tip. Following this tip gave me picture again. My only issue now is that now I’m stuck at 60hz and I used 100hz. Hopefully Apple will fix this in the next update.
I have the same issue with my external monitors after installing this new Tahoe update. How do I uninstall this update? I don't have time to try and figure out what is happening.
macOS 26.1 changed nothing on my end. My 700€ monitor is still useless..
same issue here after updating to macOS 26.1
problem exist on my Mac Air M2 using an USB C cable via Thunderbolt. Tried various Monitors but the problem is on every monitor I tried
And using an active TB3 cable doesn't work as well.
I have the same issue after upgrading to 26.1. I have a 49" Samsung G9 connected via a Kinston Thunderbolt dock. It worked fine up until I updated to 26.1 less than 5 minutes ago.
✅ Solution 2. Deep System Reset worked! Thank you abhiraj_chatterjee
External Monitor Not Detected After macOS 26 Upgrade
