External 4k monitor not working after macOS 15 Sequoia upgrade

Hello,

I had the same issue. My HDMI to USB C cable worked perfectly until I updated to Sequoia. To verify that it was an OS issue, I tested the same cable with my wife's computer that was still on Sonoma and it continued to work normally. Just replaced my perfectly working HDMI to USB C cable with an Anker brand cable and now it works. My guess is that the new OS only supports certain name brand cables. Hope this helps.

Same issue with me, although timing feels slightly off, I have 2 Mac books, an M1 and M2, they are connected to a Samsung monitor via a USBC4 cable which provides power and downstream second monitor and hub connectivity... both Macbooks worked on the day I upgraded to 15.0.1 and have worked withe monitor since, but both have now, on the same day, stopped working with the monitor except, except for receiving power. The same cable is in use, and testing it with other devices shows it can provide display and downstream device connectivity when I plug it into a iPhone, iPad, PC, and chrome book.

Same, except it worked for 24h. When I accidentally unplugged the power to the external display the screen went off. When I powered it back-up the monitor came back 'online' the MacBook seems to detect the screen, but it remains blank and does not show up in the display settings. When pluging the HDMI, the macOS display resolution changes.

Funny enough, I tried my ipad Pro as screen, then suddenly the second HDMI screen came to life !!

I am getting 3 distinct displays now. As soon as I diconnect the iPad, the external display goes dark again.

Wildest thing ever, when i log out of my user, the second HDMI display come back online, showing two different sized desktops - as if its a 'echo' of the session when all 3 where running.

Something in Seqoia is very broken !!

Same issue. I have a 2nd monitor, Samsung 27" ViewFinity S7 (S70D) using usb-c to DisplayPort and it was working great until upgrading from Sequoia 15.0.0 to 15.0.1. Same problem on both my Mac mini M2 Pro and my MacBook Pro M3 Max. I ordered a usb-c to HDMI cable to see if that might fix the problem.

I'm having the same issue. My external 4k monitor worked fine when I updated to Sequoia from Sonoma. Today I updated to 15.0.1 and now my 4k monitor no longer works on my MacBook Pro M1.

Update: the new usb-c to HDMI cable works for the S70D, but the Mac mini M2 Pro will only set its refresh to 30Hz.