MacBook Pro M1 Pro issue with dual display after updating to macOS Sequoia 15.1

Also having issues similar to the original poster since 15.0.1 to 15.1 upgrade on a 2020 M1 MacBook Air. Using a Dell 27" external display. My M4 Pro MacBook Pro does not appear to have the same issue (also on 15.1). Both difficulties getting it to send output via USBC to the monitor (monitor detects no signal) and the random external screen going black during use (intermittently).

Nope, it's still not detecting.

I just updated to 15.1.1 and am using the Apple Thunderbolt, connecting with the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter. It was working perfectly fine until they released Sequoia 15.

Very frustrating.

I just updated to 15.1.1 hoping that this would be better... but it's even worse! It won't display ANYTHING now (though when I plug the HDMI directly into the MacBook it's fine). And the Anker Dock Manager software now can't even recognize the dock even though it is currently charging the MacBook. Help!

never had a problem until i updated this morning to macos 15.1.1 and my external monitor that until yesterday used to work like a charm is not even detected by my mac M1 Pro anymore...

needless to say this is overly frustrating.

[EDIT] after A LOT of restarts (turned off the external monitor as well for a couple of minutes), the external monitor started working again

Found a temporary (not ideal) workaround on Reddit:

Basically go to your display settings and manually turn down the refresh rate for your external monitor. On Reddit people who had 144Hz and above weren't getting it to work, but turning it down to 120Hz brought it back to life. For my Dell monitor, I had to turn it down from 60Hz (my maximum) to 30Hz, which looks jittery and terrible, but at least it's kind of working. Hoping this will be fixed in the next patch because it's really bad.

We are all users like you.

To let Apple know, report the problem using the feedback page:

https://www.apple.com/feedback

Similar situation. I have a Dell U4924DW. Since Sequoia 15.1.1 the monitor is not detected at all.

Incredibly frustrating.

Is the issue fixed for anybody on Sequoia 15.1.1?

And by the way, it is not only limited to M1. I have a Macbook Pro M3 and facing the same thing...

This problem persists from day 1 of v15.1 when it was in early beta. I thought may be beta software issue & reverted back to public releases. But it seems like Apple has never fixed this issue for 15.1

I have a single Monitor hooked via DisplayPort to a USB-C Hub, which is connected via Thunderbold to my MacbookPro.

I face the same issue, that the monitor is going blank for a view seconds. Do you think it could help to switch to the HDMI Port? Or would I need to connect the mpnitor directly to the Macbook, which I wouldn't prefer.

Thanks for listening

Exact same issues here too!

I've been using the Apple Thunderbolt Display since 2016, and it's been fantastic.

However, it's recently started acting up. It began intermittently working and then stopping after the release of iOS 16 (Sequoia). I'm using an M1 Max chip, and the display worked perfectly for the past year or so.

Strangely, when I connect it to my older Mac Pro, it works fine.

I don't understand why it's stopped working with my M1 Max MacBook Pro.

I've heard that replacing the Thunderbolt cable (not the power cord) might fix the issue.

Do you have any suggestions on how to resolve this?

I've contacted Apple support in Korea, but they seem clueless about the problem.

Finally the issue for me is solved. Instead of using a HDMI to USB-C adapter I purchased a DisplayPort 1.4 to USB-C adapter connected to the Dell G3223Q 32" and no more issues. I also deleted the display preferences files in MacOS:

/Library/Preferences/com.apple.windowserver.displays.plist

/Users/<YourUser>/Library/Preferences/byHost/com.apple.windowserver.displays.<ALPHANUMERIC_CODE>.plist

In the last case, it can be multiple files.

And then restart. I hope it can help

That's why it seems to be issue with TB4 Docks. I have 2 screens connected to my dock. 1 Dell 27" 4k Screen with USB-C to Display port (this one works fine) and HDMI to HDMI on the 2nd screen, which is LG 27" 2k screen (this one flickers). Sadly, the Anker TB4 Dock only offers HDMI Out for the 2nd screen, hence I can't get around this issue.

I really hope Apple fixes this, the issue is really annoying.