Blue flashes on external monitor


macrumors newbie
Original poster
Apr 3, 2010
This is killing me, I upgraded to Catalina on my 2017 iMac and my external monitor is flashing blue intermittently. I am using a USB-C to HDMI adapter and then an HDMI cable to my monitor. I have tried resetting SMC, resetting VRAM, and creating a new user account. All have done nothing to solve my problem. I also tried booting into safe mode, but it wouldn't access the external monitor.

I made a short video to show the problem here:

I asked around and the only solution is to revert to Mojave which means I'd have to spend a lot of time re-setting up my computer and I'd really prefer not to do that. I also know I'll have to upgrade again at some point anyways and I prefer not to be stuck in the past.

I bought a cheap USB-C to RCA cable thinking I could just use the monitor's RCA connection instead but that didn't work. It also didn't work on my Windows 10 laptop with this monitor or my TV so I suspect the problem is the cable.

Any non-obvious solutions I should try?
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