Fix for External HDMI monitor with Mojave 10.14.5?

SOLVED!!!! MANY THANKS!!!!

Hello again... well - went to the Apple Store at nearby WIllow Grove Mall and purchased a Mini DisplayPort to HDMI adapter. This goes to a female HDMI port so I need to use an HDMI cable with it to hook up the Samsung Smart TV.

This fixed my intermittent issues and seems rock-solid so far! I am thrilled to have my nice big screen back, just in time to be starting a new job with a company that uses Apple-based tools for system configuration!

Many thanks for all your kind help! And I hope Apple finds it in their heart to forgive me for ever doubting Mojave!!! Seems like it truly was a coincidence that my Mini DisplayPort to HDMI cable went bad right around the time I upgraded to Mojave. (Proof: The old cable also failed on our Mac Mini that is still running under High Sierra).

So I remain a lifelong die-hard MacHead...

Hello Luis and thanks for ideas! I just tried the MacBook and cable with a different Samsung TV, and could not get the Mac to "see" the display (does not show up in System Preferences / Display. Tried repeatedly pressing OPTION and clicking on "Detect Displays" - no joy.

Then on a hunch I sprayed Contact Cleaner in the cable connectors on each end. It is working perfectly now! I now wonder if it was a coincidence that it stopped working around the time I upgraded to Mojave. I am cautiously optimistic... will see how it goes. In any event many thanks for getting back!

Oh well... sorry to say that this too may have been a coincidence! After spraying Contact Cleaner on the Mini DisplayPort to HDMI cable, the external monitor worked fine for quite a while. I could sleep and wake up the MacBook Air and the Samsung TV still worked perfectly (in closed clamshell mode)!

But after going away to do some chores and coming back, the Samsung TV was once again indicating No Signal. Tried powering down the TV, then unplugging and re-plugging the cable. No Joy. Simply left the TV powered up (with the proper HDMI port selected) and after a few minutes the MacBook Air screen went black, then when it came back both MacBook Air screen and Samsung TV were working perfectly again.

So sadly this is some kind of weird intermittent. Still investigating. Wonder if Apple has any bug fixes in the works for external displays for Mojave???

Hello -- well, that is encouraging. I had no problems at all until upgrading to Mojave. I'm wondering if Mojave may possibly have some incompatibility with older MacBooks (mid 2012)?

This seems to be a truly intermittent issue. This morning, I cannot get the MacBook to "see" the Samsung TV at all. Tried powering down the Samsung TV, pulling mini-DisplayPort plug, plugging it back in, powering up Samsung TV. Nope. Tried rebooting, Nope. Pressed OPTION while in Display Preferences and tried to recognize the display, Nope. Tried a different HDMI port on the Samsung TV. Nope.

It is acting as if the Samsung TV is not there. Samsung TV works perfectly with a Raspberry Pi system (but this uses a different HDMI <-- > HDMI cable). Really frustrating because I may be granted a web design gig and the big monitor is a huge help. I do not want to revert to High Sierra unless absolutely necessary. Hoping that Apple will fix it, but I don't think a lot of effort could be justified to support old MacBook hardware. Will try a new cable when I can... Thanks!