iMac external monitor issues

I run a pair of external monitors on my 2017 27" iMac. This morning, one of them was dead, no image, didn’t show up in Displays in System Preferences, nothing.

First troubleshooting step was to plug each external monitor into the other Thunderbolt port on the computer. As soon as I do that the previously dead monitor springs back to life and the other one goes black. This seems to be an issue with the iMac hardware, rather than the cable or the monitor. That’s not good.

Oddly, the port will still do a USB-C data connection just fine. I can plug a USB-C drive into it and it pops right up. Only video out seems to be a problem.

I’ve tried every which way to get this port working. I’ve been running the monitor on a USB-C to DisplayPort cable, I figure maybe a different type of connection might coax it to life. Two different USB-C to HDMI adapters later, no joy. I even swapped the Dell 4K for an LG UltraFine 4K with a straight USB-C connection. Still no luck.

Of course, this would happen at a really bad time to buy a new 27” iMac.

Since I run two external monitors with my iMac, arguably losing one of them isn’t a big deal. However, I really like being able to put different support material on either side. For instance, when I’m on a video call I’ll often have Teams full screen on the iMac, with my notes on the left monitor and a document on the right monitor.

Casting about for solutions, I played around with Sidecar on my 12.9" iPad Pro. It would work, but if anything I use my iPad more often than I use my computer. I don’t really want to dedicate it as a display. My current hack for the second monitor is to hook up an old Apple TV HD to the Dell 4K monitor and AirPlay to it as an external display. It works, after a fashion, but it’s too laggy and blurry to be a long-term solution.

This would actually be a good use case for Universal Control. I could hook up my MacBook Pro to the monitor and use one keyboard and trackpad to control both. Unfortunately, at the rate they’re going we’ll probably have new iMacs by the time Universal Control is out.

Right now, I’m looking at solutions to run two displays through a single Thunderbolt port. I could get a dual display adapter like this one. However, I’m planning to get an Apple Silicon iMac as soon as the big one comes out. After I replace the iMac I wouldn’t really have any use for the dual-display adapter. Another possibility would be an OWC Thunderbolt dock. That’s considerably more spendy and I don’t need the additional capability it offers to solve this particular problem, but I was already considering one of those for my MacBook Pro, so it would potentially have a use beyond my current iMac.

Any thoughts from the MPU community for either additional troubleshooting or other workarounds?

Just to be thorough, I would reset pram and smc.


Good thought on PRAM and SMC. I gave both a try. The monitor flickered briefly back to life when I hooked it up after the PRAM reset, but it went back to black after less than a minute. Definitely feels like a hardware problem.

I went ahead and ordered the OWC Thunderbolt dock.

I doubt very much it’s software, but worth logging in as a brand new admin user to see if it’s the same?

Ah. Too bad.

Good idea with the dock, I was going to recommend that. You will probably need one anyway.
Also does this monitor support daisy chaining the other monitor?

Got my OWC Thunderbolt dock today. Hooked it up to the iMac and plugged both external displays into it via USB-C to DisplayPort cables. Both of them fired right up, no problem. Nice to be back to having more screen real estate (though it was the asymmetric aspect of only having one external display that really bothered me).

Hopefully, this should tide me over until we have new iMacs.


Does it treat them as 2 separate external monitors or one single display over 2 monitors?

Two separate monitors. Just like as if I had plugged each monitor into a Thunderbolt port on my iMac (when both TB ports on my iMac were working).

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Do you have aftermarket RAM in the iMac? Something similar happened to my iMac Pro and, as odd as it sounds, the RAM was to blame. (It had worked fine for a long time until it didn’t.)

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