Apple M1 Max No Longer Supports Dual Monitors Simultaneously

Question

Level 1

8 points

Apple M1 Max No Longer Supports Dual Monitors Simultaneously

I’m encountering an issue with my Apple M1 Max (OSX 15.3.1 (24D70)) where dual external monitors are no longer working together, even though they worked fine before.

Issue Details:

When I connect one monitor via HDMI, it works perfectly. However, when I connect a second monitor via USB-C, the HDMI output stops working.
The same issue occurs when using two USB-C monitors—only one will be active at a time.
Each monitor functions normally on its own, regardless of the port used. The issue arises only when both are connected simultaneously.

Troubleshooting Steps Taken:

Reinstalled macOS (without formatting the disk), but the problem persists.

Since this setup used to work without issues, I’m unsure what caused it to stop functioning. Has anyone else experienced this? Any insights or solutions would be greatly appreciated!

Thanks in advance!

MacBook Pro 16″, macOS 15.3

Posted on Feb 13, 2025 08:35 PM

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Answer 1

Feb 14, 2025 12:24 AM in response to Carlos_I

Mac Studio (2022) - Technical Specifications - Apple Support

"Simultaneously supports up to five displays:

Support for up to four Pro Display XDRs (6K resolution at 60Hz and over a billion colors) over USB-C and one 4K display (4K resolution at 60Hz and over a billion colors) over HDMI"

Which monitors are you trying to use with the Mac Studio?

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Answer 2

Feb 15, 2025 02:12 PM in response to Carlos_I

The Samsung appears to be a 21.5” HDTV with a resolution of 1920x1080 pixels, and two HDMI inputs.

https://www.bhphotovideo.com/c/product/820769-REG/Samsung_T22A350_T22A350_21_5_350_Series.html

The LG appears to be a 27” 1920x1080 monitor with a maximum refresh rate of 144 Hz, a DisplayPort input, and a HDMI input.

https://www.lg.com/sg/consumer-monitors/gaming/27gn65r-b/

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Answer 3

Carlos_I Author

Level 1

8 points

Feb 15, 2025 01:23 PM in response to Servant of Cats

I’m using a MacBook Pro with an Apple M1 Max.

The external monitors are a Samsung SMT22A350 and an LG UltraGear 27GN65R.

I’ve been using this setup for about two years.

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Answer 4

Feb 15, 2025 02:15 PM in response to Servant of Cats

Since these displays have different manufacturers and models, I don’t think we are seeing the issue where manufacturers sometimes cheat, and use the same “unique” ID for two displays.

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Answer 5

Feb 15, 2025 02:25 PM in response to Carlos_I

macOS 15.3.1 came out just the other day (2/10/2025). So that’s one recent change to your configuration. That doesn’t necessarily mean that it is the one that broke things, but it is a change.

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