Does Catalina support Target Display mode?

I would try resetting NVRAM and SMC and see if that helps.

FWIW, I am using a 2011 iMac (running High Sierra 10.13.6) as a display for my 2014 MBP (running Catalina), and it works just as it did in Mojave.

The most obvious difference is the TB3->TB2 adapter (my mac still has TB 2 so I only use the Thunderbolt cable), but that should still work.

Just to be absolutely clear: the iMac that is to be used in Target Display Mode must be running High Sierra or earlier.

The other mac that will use the iMac’s display CAN use Catalina. That is the configuration that I’ve been using with no issues whatsoever.

Hi drshwartz

good day there ,I have just refresh my iMAC 2013 to Catanila and I trying to using TDM with my MacBook and Lenovo , neither of them are working .

i have THB connectors . Still not work , could you please kindly let me know how should O do to make it work

Unfortunately it appears that an iMac running Catalina can no longer function in TDM.

A Catalina mac can use the display an iMac with an older OS that is set to TDM (something I do daily with my Catalina MBP to my High Sierra iMac).

I don’t know if this is lack of support in Catalina itself or if installing Catalina updates firmware and that kills support. If it is the former it may be possible to revert to Mojave or dual boot for the times you want to use TDM. If it is the firmware, then that is probably impossible to get it back.

Two things that I noted in the article you linked:

a) It only mentions iMac models up to 2015, which suggests the original text is rather old. It says the article was "updated" in November 2019, but it is unclear what was updated. There is no mention of Catalina.

b) There is one incorrection in the article. It says

"Your iMac should automatically recognize the presence of a digital video signal at the Mini DisplayPort or Thunderbolt port and enter Target Display Mode."

This is (fortunately!) not true. You have to explicitly press Command-F2.

I always have the Thunderbolt cable connected between the MBP and iMac, but I do not always want to use TDM. Often I want to do something else on the iMac.

Just to clarify: is your iMac working in TDM running Catalina? Or just the mac using the display? AFAICT the first does not seem possible, so it is important to understand what is what.

Please elaborate.

The problem is not with the iOS version in the MBP, but with the version in the iMac. I updated first the MBP, and I did not have any problem; then I updated the iMac and the display mode stop working. Apple support says that the version in the iMac has to be High Sierra or earlier, "You can't use target display mode with later versions of macOS". Do you know why they discontinued this mode? It is very useful, we do not have space to have 3 screens in our desks.

"Make sure that your iMac is using macOS High Sierra 10.13.6 or earlier."

(From https://support.apple.com/en-ca/HT204592)

Which versions of macOS broke Target Display Mode? Whether TDM is supported or not depends on the models of Macs involved, not the OS.

There may have been a bug in a particular point release of High Sierra. I can't say.

FWIW, I would have been happy to test Target Display Mode with Catalina, unfortunately I don't have an iMac old enough to try that's nearby.

FWIW, I have a 2011 iMac that, as you said, cannot go past High Sierra, and a 2104 MBP running Catalina.

Target Display Mode works just fine.