MacBook Pro M1 Pro dual monitors not working

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Samuel_F-V Author

Level 1

4 points

MacBook Pro M1 Pro dual monitors not working

Dual external monitors suddenly stopped working.

I have a 16" Macbook pro with M1pro chip. I bought it because it supports dual external monitors.

A couple of weeks ago I upgraded to Tahoe 26.1 and everything was still fine.

Then after a work trip it stopped working.

One monitor is connected with HDMI and the other with USB-C to DP.

Both screens are working if they are the only one connected and i have tried with different cables without any changes.

If i have one external monitor connected (doesn't matter witch or what cable) It works with that screen. But if I connect the other one it always switches to only my right monitor.

I've tried switching cables and USB-C ports and it doesn't matter.

I also tried upgrade OS to 26.2 but with no success.

Here is a video on the issue:

https://youtu.be/RspBflv5GuQ

Anyone knows what the problem might be?

I also have a Mac-mini that I sometimes connect to the left screen but I can't see how that would make it "locked" to that computer. It has worked to switch between them before.

MacBook Pro 16″, macOS 26.2

Posted on Jan 15, 2026 6:31 AM

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Answer 1

Top-ranking reply

Grant Bennet-Alder

Level 10

141,901 points

Jan 15, 2026 8:54 AM in response to Samuel_F-V

<< The USB-C to DP cable I use is this: https://amzn.eu/d/d9ihiwJ and it has worked fine even if it's 1,8m long, from May last year until last week. It still works but not just at the same time as with the HDMI Cable. >>

Lots of folks can't believe their just-below-spec or slightly-too-long cables that seem to work in other situations and other versions of macOS no longer are reliable under recent macOS.

But after they try cables that are inside the specs, MANY post back and tell me they actually make a difference.

Try a better cable. The cost is not very high.

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Answer 2

Jan 15, 2026 7:22 AM in response to Samuel_F-V

that appears to be a 1920 by 1080 display at 8 bits/color with refresh rates up to 75 Hz.

interfaces include

HDMI 1.4

DisplayPort 1.2

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.

Cables "shipped in the box" with most displays, and cables "found" behind your TV set are immediately suspected of being under spec and inadequate, unless they are clearly labelled as certified and up to the standard.

--------

Display port adapters from USB-C are limited in length to one meter maximum, unless the bulge of the electronics is in the middle of the cable. In that case, one meter maximum of USB-C cable leading INTO the converter electronics, and one meter of DisplayPort cable leading OUT of the converter electronics is acceptable.

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Answer 3

Jan 15, 2026 11:16 AM in response to Samuel_F-V

Todays high end displays are running close to the top possible speeds that kind of logic can support, using 3 Volts push-pull signals, without generating an occasional error.

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video. Folks are really producing commercial feature films on Apple equipment, so It MUST meet the highest standards. That can not tolerate any dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or Transmit errors due to low-spec or overly long cables.

More recent versions of macOS have become increasingly picky, and if an error occurs, the display is dropped.

HDMI cables (or the HDMI portion of a cable/adapter) use higher (5 volt) levels, and can be modestly longer.

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Answer 4

Samuel_F-V Author

Level 1

4 points

Jan 15, 2026 8:21 AM in response to Grant Bennet-Alder

Ok, I checked my cables. The HDMI cable is "Premium" as your specs above. I used to have another branded EKON that is 8K and ethernet. It worked before with the EKON cable and I tried yesterday to change to the "premium" cable with no success.

The USB-C to DP cable I use is this: https://amzn.eu/d/d9ihiwJ and it has worked fine even if it's 1,8m long, from May last year until last week. It still works but not just at the same time as with the HDMI Cable.

As you can se in the settings menu for monitors only one external monitor is showing even though both are connected.

And in this photo you can se that the other monitor is on (it's brighter in the corners), and not saying "no signal". If its not connected it says "no signal".

I really don't believe it's the cables that is causing the issue.

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Answer 5

Jan 15, 2026 7:05 AM in response to Samuel_F-V

What makes and models of displays?

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Answer 6

Samuel_F-V Author

Level 1

4 points

Jan 15, 2026 9:15 AM in response to Grant Bennet-Alder

I'm not sure why you say that USB-C to DP cant be longer than 1m? I've looked around and most places say that 2-3 meters isn't a problem. And I find it really hard to find a cable that's just 1 m long.

I mean, the specs on the cable are ok and everything worked for 6+ months without any problems. And both cables still work, just not at the same time

But I'll get a new cable tomorrow ad let you know how it works.

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Answer 7

Samuel_F-V Author

Level 1

4 points

Jan 15, 2026 7:09 AM in response to Grant Bennet-Alder

Acer VG270, 2 hdmi inputs and 1 DP.

Both are around 6 month old.

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MacBook Pro M1 Pro dual monitors not working

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