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

Question

Level 1

13 points

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

Reply

Answer 1

Top-ranking reply

eccevery Author

Level 1

13 points

Aug 29, 2025 12:33 AM in response to Grant Bennet-Alder

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.

Reply

Answer 2

eccevery Author

Level 1

13 points

Aug 29, 2025 09:52 AM in response to Grant Bennet-Alder

Grant Bennet-Alder wrote:

<< 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.

Allright, I will try that. It does not make sense in my head though... The monitors are not daisy chained, there should not be more data going though the cable just because two are connected to different TB ports.

Reply

Answer 3

Aug 28, 2025 06:34 PM in response to eccevery

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.

Reply

Answer 4

Aug 29, 2025 10:19 AM in response to eccevery

<< Allright, I will try that. It does not make sense in my head though... The monitors are not daisy chained, there should not be more data going through the cable just because two are connected to different TB ports. >>

You are running these display as fast as perfect Thunderbolt Cables can reliably go. The data rates for these displays at these settings are around 38 G bits/sec

ThunderBolt cables running at full speed (that are longer than 0.5 meters) are subject to signal degradation, and modern Macs are completely unforgiving about that.

Reply

Answer 5

Aug 29, 2025 04:35 PM in response to eccevery

How are you connecting these monitors to the Mac?

Macs do not support DisplayPort MST, so you cannot daisy-chain two non-Thunderbolt monitors – even if they have ports for doing that. You also cannot connect two monitors to a plain USB-C hub or dock. If you are going to hook up two monitors to a single hub or dock, it needs to be a Thunderbolt hub or dock.

These particular monitors have high enough resolution that it may not be realistic even to daisy-chain two of them using Thunderbolt.

Reply

Answer 6

Sep 2, 2025 07:47 PM in response to eccevery

The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

Reply

Answer 7

eccevery Author

Level 1

13 points

Aug 28, 2025 01:53 PM in response to leroydouglas

No dock. Directly into the macbook.

Reply

Answer 8

eccevery Author

Level 1

13 points

Sep 3, 2025 12:02 PM in response to eccevery

Now I tried with Apples own TB5 Pro cables - same issue. Its not the cables.

I dived into the logs for WindowServer and other display-related services. It seems to me like MacOS detects the display, it shows up in the System Report and it shows the correct connected TB port, model name and unique UID. But it is never listed as a Display, besides some power delivery talk there is no communication at all over the TB cable.

This smells like MacOS bug to me.

Reply

Answer 9

Aug 29, 2025 09:28 AM in response to eccevery

<< 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. >>

ChatGPT is not smart. ChatGPT can only report the most common writings, and not the correct ones. In this case that answer reflects that the majority of users are clueless about the correct answer, is that answer is COMPLETELY Wrong.

--------

How many displays are supported depends on specific Apple-silicon M-series processor type { Plain, PRO MAX or ULTRA} and Not on number of processors at all.

As you can see from the specs you looked up, Your PRO processor supports up to TWO external Fully-Hardware accelerated displays (three for Mac mini).

Reply

Answer 10

eccevery Author

Level 1

13 points

Sep 3, 2025 01:44 PM in response to eccevery

I have now also tried Apples own TB5 Pro cables - same issue.

MacOS detects the second monitor on the Thunderbolt bus, the display model shows up along with vendor ID, UID etc and I can see power delivery negotiation in the logs - but it is never once detected as a usable display. Either this is a MacOS bug or it does not for some reason support these specific monitors or resolution in pairs. Looks like DisplayLink wont work either, there is no chip that I find that supports dual 5K2K monitors.

Guess Im down to one monitor after all.

Reply

Answer 11

Sep 3, 2025 04:06 PM in response to eccevery

did you try one ULTRA HDMI cable of modest length?

The Mac provides slightly more bandwidth for its direct HDMI port than even for a Thunderbolt-4 port.

Reply

Answer 12

Aug 28, 2025 12:54 PM in response to eccevery

eccevery wrote:

• 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.