External monitor support stopped working on MacBook Pro M3

Question

External monitor support stopped working on MacBook Pro M3

I have a MacBook Pro M3 Pro running 26.2. I have a portable USB monitor that has been working fine for some time. Normally I just plug it in an I get an extra screen, very simple. But this has all suddenly stopped working, and I'm pretty sure the problem is on the Mac side.

Now when I plug in any screen, nothing happens. I have tried 3 different ways: directly through HDMI, directly through USB-C, and indirectly through USB-C using an HDMI adaptor. All behave identically: no second screen appears in the displays control panel. I have tried different cables, and the same monitors and cables work fine on other machines. The USB monitor powers on, but the screen remains blank.

Now it gets weird. If I hit play in a presentation in Keynote, it asks if I want to play it on the external screen (connected via any of the methods I listed), and if I say yes, it then plays perfectly on the external screen, but it still does not appear as a display in the settings panel, and the screen can't be used for anything else (mirroring or extended desktop). The mirroring icon does not appear in the menu bar (it appears in the control centre, but does nothing when clicked), and no mirroring options are shown in the displays control panel. I also note that the USB monitor does not appear as a USB device when looking at system information, even when it is connected and working.

I have rebooted with and without a second screen connected, but nothing is changing. Is there some way of resetting the monitor detection? Is zapping PRAM still a thing?

MacBook Pro 14″, macOS 26.2

Posted on Feb 1, 2026 4:59 AM

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Answer 1

Feb 1, 2026 10:51 AM in response to Marcus Bointon

I do not mean to malign you, but it would help Readers if you could fully describe BOTH data and power connections in detail rather than saying its fine.

I cannot see what your setup is and I am confused about how you are getting power to it. This has been a very common problem with these displays, as they might not be able to use a USB connection for BOTH Power and Data, due to shortcomings in display firmware when used with USB-4.

The solution for some Posters has to add an additional connection cable, which meant one could provide power while the other provided data.

I am sorry to not be able to home in on exactly what the issue is here. Your situation is complex.

Consider trying these setups in Safe mode, where almost no third-party add-ins are loaded.

"Works in Safe Mode, fails in regular mode" implies, 'Its something you added'.

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Answer 2

Feb 1, 2026 7:54 AM in response to Marcus Bointon

it is entirely possible that display is connecting as an AirPlay device, not using any of the cables you provided.

what macOS were you running when it was working normally?

Simultaneously powering and running these portable displays has been problematic, as the Display-Makers did not properly account for Thunderbolt-4 changes. How does this display get POWER?

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Answer 3

Feb 1, 2026 11:02 AM in response to Marcus Bointon

For ordinary displays, a common source of non-detection happens when cables that are under spec or too long are used.

for USB-C cables, these should be certified and labeled with a USB SuperSpeed Plus symbol or USB-10 or -20 symbol. Lengths are strictly limited to ONE meter maximum.

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive. The HDMI cables "found" behind your TV set are unlikely to be adequate.

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Answer 4

Feb 1, 2026 8:10 AM in response to Grant Bennet-Alder

As I said, I'm running 26.2.

Does AirPlay even allow you to select displays connected via HDMI? When it pops up in Keynote, it names the device it's connecting to, either the USB monitor or the USB to HDMI device, so I don't think it's AirPlay. Also I would not expect AirPlay to be a factor when connecting to a device that has no wireless connectivity at all. To be sure, I tested with wifi and bluetooth turned off, and it acts the same way.

The portable monitor is bus powered, and it's been working fine for the last 2 years. Bus power would not explain why it would fail to identify the internal HDMI connection to a normal mains-powered external monitor, which has also been working perfectly since I've had the MacBook.

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Answer 5

Feb 1, 2026 9:23 AM in response to Marcus Bointon

<< The portable monitor is bus powered, >>

if your only connection is HDMI, HDMI does NOT supply any Bus power.

To use it that way, you would need an additional cable for power, which you have not described in detail.

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Answer 6

Feb 1, 2026 10:09 AM in response to Grant Bennet-Alder

I’m fully aware of that. As I said, I have tried it with other connections including HDMI to other monitors that are mains powered and have nothing to do with USB.

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