External monitor works with MacBook M2 but not Macbook M4

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Level 1

4 points

External monitor works with MacBook M2 but not Macbook M4

My company provided a new Macbook M4 and I've got an external monitor that I've been using with my personal Macbook M2 but while I can plug into mine and get the display to work fine it doesn't do the same in the newer machine. Both are upgraded to Sequioa 15.3.2.

I had to set "Allow accessories" to "Always ask" before I would see the dialog prompt asking if I wanted to allow it to connect but it would only give me about 2 seconds before the dialog would disappear. If I set it to always allow accessories connect it still doesn't detect the external display.

I've hooked up a powered monitor when I've been in the office without a problem, the only difference is that this one is powered via the USB-C cable from the computer.

Everything is the same between the computers except for the processor. Any other suggestions? I really need to be able to travel and have a two monitor setup, development is just frustrating otherwise.

MacBook Pro 16″, macOS 15.3

Posted on Mar 29, 2025 11:25 AM

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Answer 1

Mar 29, 2025 9:40 PM in response to captaingeek

Although there are some causes that might be affecting multiple monitors, I would not assume that the root cause must be the same for EVERY portable monitor.

Some things that come to mind:

It is quite possible that MBPs vary in their willingness to provide "unadvertised" amounts of power and that the new ones offer less of it.
Portable monitors themselves vary in power requirements – which implies that it's possible that some might be able to run off power from a notebook, while others might require an external power supply.
There also may be breakdowns in negotiations between the MBP and portable monitors – either over power, or over video, or both. This could happen if one side or the other is using code which checks for compatibility in a too "brittle" way. MacBook Pros with M4 Pro and M4 Max chips have Thunderbolt 5 ports with DisplayPort 2.1, and possibly the higher versions are confusing firmware in external monitors.

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Answer 2

Mar 29, 2025 4:33 PM in response to captaingeek

Portable displays firmware issues have turned out to be a substantial issue. Current thinking is that these display makers are not adhering to standards for Thunderbolt-5.

current work-arounds include badgering the display-maker for a firmware fix. (one maker responded and got their displays working)

providing an additional USB cord for power

using an external power supply for the display.

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Answer 3

captaingeek Author

Level 1

4 points

Mar 30, 2025 10:18 AM in response to captaingeek

I've now tried a couple of external power supplies for the monitor but it doesn't help. Though the MBP does get power through the cable so it's not using the battery.

I'm going to switch to an HDMI cable and see if that will allow me to do what I need with the resolution I expect. Thanks!

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External monitor works with MacBook M2 but not Macbook M4