What to do when MacBook Pro M2 only recognizes one monitor?

I have two Dell U4025QW connected via TB4 to my Macbook M2 Pro 14 inch laptop. Only one of them works, whichever I plug in last wont work. According to the specs my laptop should support up to two external monitors at 6K and 60 Hz via TB4, mine are 5K.

I have tested the usual suspects like

Lowering resolution and refresh rate
Unplug and reconnect
Reboot
Deleted the display plist file and reboot

Any ideas?

[Re-Titled by Moderator]

Original Title: Only one external monitor work at the same time

Posted on Aug 28, 2025 12:24 PM

Top-ranking reply

eccevery Author

Level 1

13 points

Posted on Aug 29, 2025 12:33 AM

Grant Bennet-Alder wrote:

HDR=ON and 60 Hz only fits on a Thunderbolt cable if all devices support Display Stream Compression (DSC).

Did you try HDR=OFF and 60 Hz? That can save around 15 percent bandwidth.

Also your ThunderBolt cables should be 0.5 meters or shorter. longer cables may not be reliable.

Those displays support HDMI as well, and because the data rate is slightly Higher than the maximum 40 GHz over ThunderBolt, M2 and later Macs make a special case and provides 42 GHz for Direct-connect HDMI displays, Provided you use a certified ULTRA HDMI cable of modest length. Other types of HDMI cables will NOT suffice. Adapters will not suffice.

I don't seem to have an option to turn HDR on or off. When I search in settings I find the menu item to the left, but there is no toggle for it. I have tried lowering resolution and refreshrate all the way down to 30 Hz, closing the lid etc but so far nothing has worked.

The cables are the ones that comes with the monitors. They are 1 meter long each. Should I try switching them out for a shorter one?

I have tried HDMI as well, also with the cables that comes with the monitor, and I get the exact same issue. One works at a time.

Both are detected though. When I plug in the second one that monitor wakes up, scans for signal and then says it has no signal from the thunderbolt device. I can see both of them in the system report utility.

ChatGPT says it must be a 12-core CPU for it to work, I only have 10 cores, but I cant find any doc at Apple that supports that theory.

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16 replies

Aug 29, 2025 09:18 AM in response to eccevery

<< The cables are the ones that comes with the monitors. They are 1 meter long each. Should I try switching them out for a shorter one? >>

Cables "shipped in the box" are notorious for being "lowest bidder" cables, and Readers Often find they are NOT adequate for the most extreme resolutions.

ThunderBolt cables running at top speeds (as these will be for these displays) are limited to 0.5 meters maximum length. Longer cables may be unreliable, and the first burst of transmission errors will throw the display offline.

So yes, buy at least one Thunderbolt cable (with a ThunderBolt logo on each end) and 0.5 meters or shorter.

<< I have tried HDMI as well, also with the cables that comes with the monitor, and I get the exact same issue. One works at a time. >>

As above, "shipped in the box" cables are often not good enough. The advantage of using the higher data rates for HDMI can ONLY be attained when ULTRA HDMI cables are used. These cables are completely different from all older HDMI cables (but are backward-compatible). The length is not specifically limited, but it makes sense to limit them to around 1 meter or less.

So yes, buy a certified ULTRA HDMI cable.

What to do when MacBook Pro M2 only recognizes one monitor?