MacBook Pro M3 Pro — Second external monitor not detected after macOS Tahoe update

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Level 1

32 points

MacBook Pro M3 Pro — Second external monitor not detected after macOS Tahoe update

Hey everyone,

Since June 28, 2026, I've been unable to use two external monitors simultaneously on my MacBook Pro 14" M3 Pro (macOS Tahoe 26.5.2). It was working perfectly before.

Setup:

MacBook Pro 14" M3 Pro, 18GB
2x LG Ultrawide monitors connected via Thunderbolt

Behavior:

When both monitors are connected, macOS only detects one (LG2). The other (LG1) shows a frozen last frame — it still receives power but macOS stops sending new frames to it. macOS always prioritizes LG2 regardless of which Thunderbolt port it's connected to (tested by swapping cables). Each monitor works perfectly on its own.

When LG2 is disconnected, the screen reorganizes immediately — meaning macOS detects the electrical connection/disconnection but doesn't activate it as a second display.

What I've tried:

Different Thunderbolt ports (all combinations)
Native HDMI port + Thunderbolt
Deleted display preference plists
NVRAM reset
Restarted with both monitors connected
Safe mode

Nothing worked. This is clearly a macOS regression — both monitors worked fine until June 28.

Already submitted feedback to Apple (FB23495201). If you're experiencing the same issue, please duplication my report in Feedback Assistant to help prioritize the fix.

MacBook Pro 14″

Posted on Jun 30, 2026 10:00 AM

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Answer 1

Top-ranking reply

Dieges Author

Level 1

32 points

Jun 30, 2026 11:00 AM in response to Dieges

SOLUTION FOUND — BetterDisplay EDID override

After extensive troubleshooting, I found the root cause: both LG monitors report the same generic serial number (01 01 01 01) in their EDID data. At some point, macOS started using this serial to differentiate displays and, since both were identical, it would only activate one at a time (always preferring the same monitor regardless of port or cable).

Fix using BetterDisplay (free app):

Download and install BetterDisplay from betterdisplay.app
With both monitors connected, open BetterDisplay and select one of the LG monitors
Scroll down to "Exibir EDID" / "Show EDID" section → click "Recuperar EDID" / "Retrieve EDID" → click "Recuperar Dados EDID" / "Retrieve EDID Data" → export the .bin file
Open the exported .bin file in a hex editor and change bytes 12–15 (the serial number field) from 01 01 01 01 to any other unique value (e.g. 02 03 04 05). Recalculate the checksum at byte 127 so that the sum of all 128 bytes mod 256 = 0.
Back in BetterDisplay, go to "Carregar EDID personalizado" / "Load custom EDID" → load your modified .bin file
Enable the toggle "Aplicar automaticamente EDID personalizado após a detecção deste monitor" / "Automatically apply custom EDID after display detection"
Click "Aplicar EDID personalizado agora" / "Apply custom EDID now"
Make sure BetterDisplay is set to launch at login

After applying, macOS immediately recognized both monitors as separate displays. Both are now working simultaneously with the lid open on an M3 Pro.

Hope this helps someone!

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