M1 iMac not operating external display on macOS

rjpesb wrote:

Hard to believe that it is the cable. (I would have to order a new one to try that.)

It appears Apple has changed something with regards to the ports and/or displays which requires a much better & higher quality cable. Many people have confirmed their display issues with the recent 14.x, 15.x, & 26.x updates were resolved by the use of a good high quality cable.

Sounds like the cable might have developed an issue then.

The M1 may be more sensitive to the issue than the Intel Mac Pro and does not manage to completely detect the device through it.

Try a different cable. Also, try restarting the Mac just to rule it out as a possible issue.

Need some more details.

What display? Make & Model.

How is it connected to the iMac exactly?

What do you see on the display exactly? Just a black screen? "No Signal"? Your Wallpaper, but nothing else?

Does the display get detected in System Settings ➜ Displays?

Provide details and context about the situation and someone may be able to advise.

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Should not be hard to order a new cable, Amazon will likely ship it overnight even in many cases.

I recommend Ugreen or Anker cables for best quality and price ratio.

Downgrading to an older macOS version may make it work again, but that requires erasing your hard drive entirely and installing the old version from scratch if you can even get an installer for that specific version. Apple tends to remove older versions when it issues new updates to them.