Retina 5K Mac not detecting second monitor via Thunderbolt to HDMI adaptor

Hello. I have a Retina 5K, 27-inch, Late 2015 and I'm trying to connect a second monitor via a Thunderbolt to HDMI adaptor. There is no response when I plug into either Thunderbolt port. I have a late 2013 Retina MacBook and when I plug into it, it reads the monitor immediately. I am using the same Thunderbolt to HDMI cable so I know that's not the issue. I have reset the NVRAM/PRAM and SMC multiple times. I have blown the port out with a duster can. Any help would be appreciated!

[Re-Titled by Moderator]

Original Title: Retina 5

iMac 27″, macOS 15.7

Posted on Dec 5, 2025 1:41 PM

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Dec 5, 2025 5:51 PM in response to jsb1964

That iMac has Thunderbolt 2 ports, with Mini DisplayPort connectors, that can double as Mini DisplayPorts.

iMac (Retina 5K, 27-inch, Late 2015) - Technical Specifications - Apple Support

I don't think there is any such thing as a Thunderbolt 2 to HDMI adapter, unless you're counting a dock, so I'd guess that what you have is a Mini DisplayPort to HDMI adapter.

I believe that there are "passive" and "active" versions of these adapters. If what you have is a "passive" adapter, it will only work if the computer has extra ("DP++") support. An "active" adapter has a built-in chip and can work with either a plain DP port or a DP++ port. I do not know if this is the issue, but am mentioning it, in case.

https://www.eaton.com/us/en-us/products/data-video-cables-accessories/usb--thunderbolt-and-lightning-cables/what-s-the-difference-between-passive-and-active-displayport-ada.html

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Retina 5K Mac not detecting second monitor via Thunderbolt to HDMI adaptor