No HDMI signal after update

I use lg 4k monitor and i got same problem.

The way that i solve this problem is boot with commend + R, to boot with recovery mode.

It works usually (some time not works), and you can restart, then monitor is work with normal condition.

yep, same issue here

I own a 4K Samsung monitor.

I had to fight to make the OS starting after the Catalina update.

Now OS starts correctly but the start screen is pink color instead of the default black

I have the same or similar issue with my 2018 Mac Mini. I have both an LG 4k monitor and a Vizio 4k TV, each with a different HDMI cable. (Both cables are rated for 4k 60Hz.) After upgrading to Catalina the HDMI signal no longer starts when booting up. Each morning I have to 1) unplug the HDMI cable, and 2) reset the SMC, before the Mac Mini will booth with any signal sent to the display. This is the case regardless of which monitor or cable I am using.

So for me, I have a samsung 4K over HDMI. Starting up, plugged into the HDMI port in the Mini, I get “not optimum resolution” warning with a black screen. If I switch the cable to my Anker usb-c hub with HDMI, then restart, it works fine. What’s really weird is, if I start plugged into the hub, then with everything on, switch the cable out of the hub and back into the HDMI port in the Mini, the display works fine. So HDMI port in the Mini IS sending a signal through HDMI, just not at start up? But the HDMI port in the hub works as it should.

I did all the usual stuff, reset PRAM ETC. Reinstalled Catalina, swapped cables, reset display settings to both scaled and default for monitor. None of these things worked. The only thing that works right now, as it should, is through the hub.

OK guys from what I've read in this thread and what I'm experiencing

it looks like an issue with the HDMI port and 4k output and Catalina.

I too, have a blank screen after switching on my mac mini 2018.

It's only when I connect it to a Sony TV 1080p i have picture.

Now, I also tried the reset cmr reset and dorsnt work for me.

Powered down, for 10 seconds and switching it on again still doesn't work 9 times out of 10!

WHEN finally i do get picture if i do a restart from within the catalina (apple logo, restart) it ALWAYS works.

There's output from the hdmi on to the 4k monitor (LG UD58A)

Restarted the mac mini multiple times. No problems

BUT if I choose shutdown and then restart, blank screen again.

I'm so frustrated.

I can't believe this was not spotted in Beta testing. Shame on you Apple...

I have the exact same issue; Mac Mini 2018, connected via HDMI to my 4K Samsung. Works once in a while, but 9/10 it just turns on with no signal to the TV. The ONLY thing I found working sometimes, is booting into Recovery Mode, and then back into macOS.

After a few days of being p-i-s-s-e-d off, I finally found this page, and now I have it running using one of my ATDs using a USB-C -> DP adapter. I guess I can use a USB-C -> HDMI adapter for the time being, but I am really surprised that Apple is not on this like white on sugar. Do they not monitor these forums???

I just chatted with Apple Support, and they organised a callback from their Senior Support team tomorrow evening. They did not have the issue listed on their system, but I gave them a link to this discussion, and we will take it from there...

FWIW. I had the exact same issue with my 2018 mini and Dell 4K monitor when I upgraded to Catalina. The only "fix" to get 4K @ 60Hz (w/HDR) for me was to use the Belkin USB-C-to-HDMI adapter that I got from the online Apple Store.

Glad I found this thread, I guess misery loves company. Same issue, Mac mini 2018 into Toshiba UHD TV (43LF621U19). Same symptoms, worked fine with Mojave, can restart fine, but if you power down nothing. I've been plugging in a Vizio HDTV, then switching to the Toshiba at the login prompt, a real pain. I'll try the scaling suggestion, but that doesn't give the resolution I bought a 4K Monitor for.

Have a Dell U2413 as a second screen connected via HDMI to Mac mini 2018. Worked fine with Mojave and initially with Catalina, now it's not being recognised. The screen says it is not getting a signal.

Update to my previous comment. Disconnecting the main screen (LG 24" Thunderbolt) and rebooting with just the Dell worked fine, then connecting the LG screen resulted in both screen working. Rebooting with both plugged in resulted in both screen working fine.

I had been experimenting with Sidecar when Catalina stopped recognising the Dell - could this be the source of the problem?

As an aside, on both Mojave and Catalina, I had to create a resource override file for RGB colour space for the Dell screen to get a decent image. Why can't Apple do this, it is/was a common enough screen.

Just sharing my experiences so far, with this issue after many hours of troubleshooting.

Its seems to be affecting anyone that has a 4k display and using HDMI connection.

This issue occurred 1/10 times when I was using mac mini 2018 and Mojave. I never gave it any thought until

after installing Catalina, then catastrophe struck.

I connected a windows laptop to the monitor using the same HDMI cable and all was well.

I immediately reconnected the same cable to the mini, and blackness.

After many reboots and smc resets, maybe it worked twice.

Important!! once the monitor finally displays video output, as long as I don't shutdown the mac mini

I can restart it as many times I like. Picture is always shown on the 4K monitor (LG 4K UD58B)

Once I shutdown Catalina, that's it, no more video output.

The only temporary solution (if you can call it that) is having an old monitor near, connect the HDMI cable

to it, and then switch on the mini. Once the Apple logo appears I can connect the HDMI cable to the 4k monitor

and all is OK.

Conclusion

For me, if buying an adapter, from thunderbolt to HDMI, will fix this issue I'm ok with that.

I'm not a power user/don't have many thunderbolt devices (yet anyway) to be worried loosing one port.

But for many others this could be an issue and I think Apple can fix this in a jiffy.

Remember Catalina has been released less than a week a go and just maybe this issue hasn't caught their attention yet.

Salute

I think I've found a temporary solution (until they hopefully will patch this) without having to reconnect my Mac Mini to a low res TV. My LG 4K display has a HDMI ULTRA HD Color feature. When I switch it off the display sees my Mac Mini. Anyone try this?

your story is exactly my painful experience now

same frustration here

I called Apple support 2 times who asked me to do SMC/VNRAM/Safe Mode, then reinstall OS again

nothing help

the HDMI still cannot show mac mini screen

the only way is using usb-c to hdmi

hope Apple can face this problem and fix it in Catalina soon !!!!!!!