My iMac (Retina 5K, 27-inch, Late 2015) won't connect to HDMI monitor or TV. Nor would it connect DVI cable to TVs. My 2011 generation iMac worked using adaptors out of the Thunderbolt port. I've tried the adaptors on my 2012 MacBook Air and it works fine.
Is there a compatibility problem with the monitors, by chance? Has anyone else had this problem?
Please read the instructions in https://support.apple.com/guide/mac-help/use-your-tv-as-a-display-mchlp1206/mac
Also, when posting we MUST know which version of Mac OS is installed to provide correct instructions. Without this vital information we are guessing.
Reset the NVRAM on the iMac and then restart in Safe Mode. Instructions for each are in:
https://support.apple.com/en-us/HT204063
https://support.apple.com/en-us/HT201262
This still assumes the MBA has identical outputs as the iMac.
Thanks for that link/suggestion. However, it is not working. I had taken it to the Apple store and they checked to see if it would work. They connected a monitor to my iMac using a Thunderbolt cord connected to each. It worked. The also ran diagnostics that showed that both Thunderbolt ports were functioning fine. Though I just tried to connect my MacBook Air to the iMac using a T-bolt cable and neither registered the other.
The iMac is operating on Mojave 10.15.6, but this issue has predated this version being loaded. It has never been able to connect to an external monitor. The Air is running 10.15.5.
When I plug the same cable and TV into my MacBook Air it works. But when I move it to the iMac it does not. And what worked in the store was a Thunderbolt to Thunderbolt connection rather than a Thunderbolt out to an adaptor to HDMI to the TV's HDMI port.
I ran safe start (just holding the 'Shift' button) and the safe start recommended to Reset NVRAM or PRAM. While the computer is starting quicker now (thank you for that), it is still not connecting with the monitor. I did get a message on the monitor that reads "Unsupported signal. Please check the device output." But now I again get "No Signal" on the monitor.
Also, I have been able to network (via Thunderbolt cable on Port 1) the problematic iMac and my MacBook Air. However, Port 2 will not connect. I've tried connecting the monitor through Port 1 but don't get a signal.
If it worked with an external monitor a the Apple Store then the issue is clearly your cabling or you forgot to change the TV's input. It either one or the other and you have to troubleshoot which.
Connecting 2015 27" iMac to TV monitors via Belkin Thunderbolt to HDMI or to DVI adaptors not working.
