iMacs not working in Target Display Mode
 

Im trying to use my old iMacs (2009, 2013) as external screens to my MacBook Pro.

The 2009 works automatically but the 2013 wont work.

I have tried to switch cables between the two, but only the 2009 works.

Any ideas?

'Target Display Mode' does NOT use DisplayLink technology. As a result, DisplayLink will not be able to assist you.

That said, Apple has very specific requirements in order for Target Display Mode to work as they describe here --> https://support.apple.com/en-us/HT204592

If that information doesn't help, I would suggest contacting Apple directly for assistance.

Any other way to solve it?
 

Is there any other way to solve it?

As I said previously, Apple has very specific requirements in order for Target Display Mode to work as they describe here --> https://support.apple.com/en-us/HT204592

A key element that jumps out from Apple's documentation is:

"The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed."

This appears to indicate that support for target display mode was dropped in macOS 10.15 Catalina.

If that information doesn't help, I would suggest contacting Apple directly for further assistance.

Ok, thanks for checking. I have all those requirements. Apple probably wont help with those old iMacs.
Will have another go at it myself :-)