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.
Make sure the monitor is powered on before connecting it to the Mac. FWIW, I have a 16" M4 Pro MacBook Pro and I'm typing this on an external display, although mine is connected via Thunderbolt (TB cable to a Stone Pro TB3 dock then a TB cable to TB3-capable LG 5K:2K display).
DaveGarratt wrote:
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
More recent version of MacOS have been increasingly demanding that there be NO transition errors. If the display in question has a very slightly faulty USB connection at the display, then it is plausible that the increased demands of MacOS 26 push it from being marginal to failed.
One reason I spend a lot of time here is to learn stuff. I want to know what display has this "known history" of USB port issues. I would like to read about it and see if there might be ways to work around that problem
.
I have had my share of USB-C cable issues in the past, and swapping cables may be worth trying, but I don’t think this is the whole answer.
There is something not right with the update. I have 2 x USB-C displays connected directly to the ports on a Mac mini M4 pro. Worked perfectly before Tahoe. Now only one monitor will connect unless I switch the other one off and on again. EITHER monitor works when the other is switched off. When both are on, the second display does not connect. Doesn’t matter which one I power on, it works if the other is switched off. Swapped cables between the two monitors and switched each off separately. The problem does NOT follow the monitor or cable or port… Can’t help feeling this is a related issue.
Hey -- I'd give it a few days until the version of Mac OS 26.1 comes out. While I can't talk about anything here, it causes the system to delete my message, I think most of us will have much better luck once this update comes out this week. YMMV :-)
Connect it to the system directly, reduce the refresh rate to the lowest possible setting, connect it to the docking station again. If it then works you keep increasing the refresh rate till you hit the problem. Repeat and stop at the refresh rate that still worked.
if you are not good at using Terminal, the same files can be found and deleted using Finder.
I posted the procedure right after the one by abhiraj_chatterjee far above.
linked here for direct access:
Delete displays preferences using only Finder
For now I have to use a separate HDMI cable to connect to the external monitor, which was detected properly.
[Edited by Moderator]
it happens with Dell U3425WE monitors. With a standard 24"inch monitor I have no problems using the USB C ports
I have the same issue. Same scenario, M4 MacBook. USB-C to USB-C 4K monitor that worked before macOS 26 upgrade. After upgrading, the USB-C provides power, so it is connected. If I use an HDMI to USB-C cable, the monitor works, but the cable does not provide power. Are there any other ideas besides the ones in the previous posts?
Just to add to the case to possibly help the devs work on a solution.
External display connected to my M1 Max MacBook Pro using the HDMI port, HDMI to HDMI, was working fine.
Until, I clicked the annoying screen button on the top right ( the one that make me feel like someone is watching my screen even though its just my own computer sharing display to my own display in front of me ), and I tapped on "Stop Extending"
This was it, there was now no way of making the screen extend to the monitor, the Mac refused to see it anymore, restarts, unplug and plug back in, different HDMI cables, nothing. Have had to resort to a usb c dongle for now, but I hope a fix comes about because this is weird.
<< Plugging in my MagSafe charger appears to completely break high speed thunderbolt connectivity. >>
That WavLink Dock is a USB-C dock, not a ThunderBolt dock, according to the maker's user manual.
Compatible with Type-C USB 3.1 Gen2 full specification PC/laptop
They appear to be using a symbol similar to a ThunderBolt symbol (genuine ThunderBolt has an arrow on the tip of the lower end) to indicate that port can supply up to 100 Watts power to your Mac. There is no claim in the specifications that the WavLink dock is a ThunderBolt device.
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If this is a Bug, it is a Bug with multiple charging methods, not Thunderbolt protocols.
I have 2 Macbooks, the one with 26 and the other I have upgraded to 26.0.1 before upgrading the external monitor worked but not after the upgrade. whereas the other mac where i still have 26, the external monitor works. so it's clearly the upgrade 26.0.1 which broke the display
Hello- yes, I did. I even spoke to their support team, as they had reached out to me. While I can't comment on the dreaded Public B-word (the system deleted my last post because I mentioned an unsupported OS version ;-) ), I'm relatively confident that the next release of macOS 26 should fix this issue. Good luck to everyone!
In Tahoe, the system preference file has a simpler name:
/Library/Preferences/com.apple.windowserver.displays.plist
The user preference file name still includes the long <UUID>. This article in the plugable knowledge base explains the problem very well.
what Processor type? {M-series (plain), PRO, MAX, ULTRA}
what make&model display?
What cables, adapters and docks are being used to make the connection?
External Monitor Not Detected After macOS 26 Upgrade