M3 MacBook Pro not detecting secondary monitor consistently

Question

tealoo Author

Level 1

4 points

M3 MacBook Pro not detecting secondary monitor consistently

Hi all,

So my current display setup is:

Having the MacBook Pro laptop display closed
Having one monitor with an integrated USB hub connected via Thunderbolt port (AORUS-FO32U2P).
- Note: This monitor has a USB Hub as well as charging. It has all my USB devices hooked up to it.
Having my second monitor connected with an HDMI -> USB cable to a second Thunderbolt port. (LG 27GL83A-B

My issue is that the second monitor rarely gets detected. Here is the observed behavior:

My primary monitor always gets detected
The second monitor never gets detected regardless of the order in which I connect the 2 cables.
The second monitor usually gets detected if I restart the laptop
The second monitor always gets detected when using my M1 MacBook Pro

My conclusion is that this is some sort of regression in the M3 MacBook Pro since it wasn't an issue with M1 MacBooks. I've also tried various cable configurations with the second monitor (HDMI -> HDMI, HDMI -> USB, DisplayPort -> USB, HDMI -> Thunderbolt Dock -> USB). All of these configurations btw works in my M1 MacBook Pro.

Is there a documented behavior change for the EDID monitor handshake in M3 vs older models? The fact that the monitor gets detected at restart but not any other time leads me to believe this is the issue. I've exhausted all options I can think of to fix the issue.

Thanks in advanced for reading this long explanation :)

[Re-Titled by Moderator]

MacBook Pro 16″, macOS 15.2

Posted on Jan 13, 2025 6:40 PM

Reply

Answer 1

Top-ranking reply

Servant of Cats

Level 8

39,555 points

Jan 14, 2025 12:54 AM in response to tealoo

Does the M3 MacBook Pro have a plain M3 chip, a M3 Pro chip, or a M3 Max chip? (Ones with plain M3 chips only support two monitors when their lids are closed, and when they are running macOS Sonoma 14.6 or later.)

Use dual monitors with your MacBook Air and MacBook Pro with M3 chip - Apple Support

Reply

Answer 2

Jan 13, 2025 6:50 PM in response to tealoo

later versions of macOS are much more fussy about any errors that are detected in transmitting data to the display. if an error occur, the resolution may be reduced or the display may go blank.

That is why it of utmost importance to have high-spec cables that are appropriately short, typically under one meter. These displays are running very close to the limits of switching speeds for the types of logic used. ANY variability can cause additional attenuation that creates an error.

Reply

Answer 3

Jan 13, 2025 6:56 PM in response to tealoo

your LG display appears to be a 2560 by 1440 display with HDMI and DisplayPort inputs.

to use it on an HDMI cable the cable must be CERTIFIED and sold with the magic words PREMIUM for up to 4K at 30 Hx, and ULTRA for higher. ULTRA is backward-compatible.

the unlabeled cables found behind your TV set are for 720p and not much more.

Reply

Answer 4

tealoo Author

Level 1

4 points

Jan 13, 2025 6:55 PM in response to Grant Bennet-Alder

Thanks for the response but I'm not sure that would be the issue, all of my cables are 3 ft, and also, the detection works 100% correct on laptop restart, it just doesn't work after that. For example, if I restart the laptop with the cables connected, the second display gets discovered fine, but if I just unplug and plug it in again it doesn't get detected. It also works 100% find in my M1 laptop which is running the same version of MacOS.

Reply

Answer 5

tealoo Author

Level 1

4 points

Jan 13, 2025 7:15 PM in response to Grant Bennet-Alder

So you're suggesting that there's some sort of signaling issue due to the cable itself? The reason I'm hesitant to blame the cable is because the monitor (with the same cable) worked 100% flawlessly with both my M1 MacBook Pro and even an older Intel Mac I used to own. It would make sense if it was always flaky but the issue only started with the M3 MacBook Pro, specifically with connecting the display with the laptop is already turned on. Connecting it works 100% fine when restarting the M3 MacBook.

Reply

M3 MacBook Pro not detecting secondary monitor consistently

Similar questions