Issues driving 2x 4K Displays (M2 Pro MBP & M4 Pro MBP)

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Level 1

4 points

Issues driving 2x 4K Displays (M2 Pro MBP & M4 Pro MBP)

I'm experiencing a weird issue with both of my MacBook Pros and my new 4K Monitors.

16” M4 Pro Macbook Pro
14” M2 Pro MacBook Pro

I have 2x 32" 4K OLED Displays (Asus PG32UCDM).

When I initially set them up today, everything worked perfectly using my TBT200 Thunderbolt 4 Dock (with both monitors connected via USB-C to HDMI cables)

However, now I can only seem to get 1 monitor to work at a time. I cannot get both working simultaneously no matter what I try.

USB-C to Both (No Dock)
- Only 1 works at a time
- Both work on their own (i.e. only 1 cable plugged into the MBP)
USB-C and HDMI (No Dock)
- Only 1 works at a time
- Both work on their own (i.e. either USB-C to HDMI plugged into the MBP, not both)
Thunderbolt 4 Dock - Corsair TBT200
- Only 1 Works at a time
- Both work on their own (i.e. only 1 plugged into the dock)

I'm completely stumped. It was working perfectly earlier today when I initially set them up and now no matter what I try I can only get 1 display working at a time, and I'm experiencing this issue on both of my MBP's.

MacBook Pro 16″

Posted on Feb 10, 2025 4:06 PM

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Answer 1

Ghostx Author

Level 1

4 points

Feb 12, 2025 9:50 AM in response to Ghostx

I've spoke with a couple of Apple Support Reps on the phone, and my issue is being escalated to the engineering team.

I've found a bit of a hack-y workaround in the meantime - for my M4 Pro Machine at least. Unfortunately it doesn't seem to work on my M2 Pro Machine.

I have an Elgato HD60X Capture Card. If I place this between my MacBook Pro and the 2nd Monitor, it gets detected and everything works as expected. I can now use both monitors at 4K60 on my M4 Pro MacBook Pro.

It's quite a strange issue. I'm by no means a monitor expert so everything I'm about to say should be taken with a massive grain of salt, but I'm assuming that some sort of negotiation happens between the computer & monitor when plugged in to determine frame rates and resolutions. I suspect that when adding the 2nd monitor this negotiation is breaking down and ultimately failing due to the much higher frame rates with these monitors (supports up to 4K @ 240Hz). This might explain why everything works fine when I add a capture card (which is capped at 4K60) in between. Again, I don't know much about monitors so this is just a random theory to be taken with a grain of salt.

Whether this is an issue on the monitor side, or the Apple side, or a mixture of both is still to be determined.

EDIT: Updating my M2 Pro Machine from 15.3 to 15.3.1 and seems that my workaround is working on that machine too now.

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Answer 2

Feb 12, 2025 10:09 AM in response to Ghostx

the next issue on my list when you have gone through:

• Mac DOES support two displays

• display detection is being applied correctly

is to look at cables. The Mac, especially later versions of macOS, is extremely sensitive to data errors, and data errors can be caused by cables that do not meet specs.

for USB-C cables, these must be Super-Speed PLUS for older cables, or USB-20 or better for newer cables.

Cable length is limited to ONE METER of shorter.

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Answer 3

Feb 10, 2025 5:41 PM in response to Ghostx

ok, next thing to think about:

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.

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Answer 4

Feb 10, 2025 4:42 PM in response to Ghostx

The same rules we learned for the M1 and M2 and M3 mostly still apply to the M4 models. Display interfaces are generated deep inside the system on a chip. How many there are depends on the exact type of Processor:

M3 (plain) like the 13-in and 15-in supports up to ONE External fully hardware-accelerated external display.

Except the M3 MacBook Air models with 13 or 15-in display, (and certain M3 plain MacBook Pro models when running 14.6 or later) which can support a second external display instead of the built-in display when you close the cover on the built-in display. This may require MacOS 14.6 or later.

M4 (plain) supports up to Two fully hardware-accelerated external displays (three for Mac mini)

M4 PRO processor supports up to Two fully hardware-accelerated external displays. (three for Mac mini)

M4 MAX processor supports up to Four fully hardware-accelerated external displays.

M4 ULTRA processor supports up to Eight fully hardware-accelerated external displays

Tech Specs now available here:

MacBook Pro - Tech Specs - Apple

¿After looking at that, should you EXPECT your MacBook Pro to drive that many displays directly?

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Answer 5

Feb 13, 2025 9:35 AM in response to mikemoose66

following the advice above, I decided to put 'something between' the Mac Studio and the 2nd display (hdmi for 2nd display). I routed it through an expansion dock (that I normally just use for extra usb inputs)

worked straight away.

cheers for the tip

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