IMac – 2015 Retina iMac Unable to Display 4k@60 Hz on My External Monitor

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Recently I purchased a 4K monitor for my Mac. I have a MiniDP to HDMI 2.0 converter and a HDMI 2.0 cable. I connected my late 2015 Retina iMac to the external monitor. I was able to get 4k@30hz on my external display. However, I was unable to achieve the 60hz refresh rate.

After some research I figured that late 2015 iMacs with the thunderbolt 2 ports should support 4k@60hz video output. I can't seem to figure out why I can't get the 60 Hz refresh rate.

When I open System Preferences, the Refresh Rate drop-down menu is gray.

Mac Configuration: iMac (Retina 4K, 21.5-inch, Late 2015)
MacOS: Catalina

I connected the same monitor to my 2018 MacBook Pro using a USB-C to HDMI 2.0 cable. My MacBook Pro was able to drive the display at 4k@60hz.

Thank you for your time and consideration!

Best Answer

The late 2015 21.5-inch Retina 4K iMac is fully able to drive a 4k display at 60 Hz. The monitor you have also lists that it supports 4k at 60 Hz.

The problem here must then either be your HDMI cable or your DisplayPort to HDMI converter.

I would suggest simply trying a different cable/converter.

The easiest to get working is to get a simple mini-DisplayPort to DisplayPort cable, as the monitor has a DisplayPort input. This way you haven't got the converter as an extra point of failure.

Related Solutions

Force external display refresh rate

cscreen

You can use the freely available Terminal app cscreen to force display settings. The app is pretty self explanatory.

brew cask install cscreen

cscreen -h
Usage: cscreen [-d <depth>] [-x <width>] [-y <height>] [-r <refresh>] [-s <display>] [-v] [-m] [-f] [-l] [-h]
           [-d <depth>]    : specifies the bit depth (bits per pixel)
           [-x <width>]    : specifies the width in pixels
           [-y <height>]   : specifies the height in pixels
           [-r <refresh>]  : specifies the refresh rate in Hz
           [-s <display>]  : specifies which display to use (defaults to main display)
           use a as the option to -s to specify the action on all displays
           [-i <displayID>]: picks a display based on CGDirectDisplayID (permanent per display)
           continue to use '-s a' for "all displays"
           [-v]            : display valid modes (use -s to specify display or nothing for the default)
           [-m]            : require an exact match
           [-f]            : forces settings (ignores safety mechanisms; USE AT YOUR OWN RISK)
           [-l]            : lists the current displays and modes
           [-p]            : sets the requested display to have the menu bar
           [-h]            : displays the usage

           Note: Using -p will change the display index so you will likely want to use -l again to show the current
           displays if you wish to use -p a second time.

IMac – Using iMac 21.5 mid 2010 as an external monitor of Macbook Pro Retina late 2013 15″

I've tried this and yes it does/can work, depending on your setup. So far I've managed to hook a 2012 MBP 13" to a 2012 21" iMac via Thunderbolt, and the cmd + F2 function works once logged in.

However, I've not managed to connect to a 2011 iMac (non-Thunderbolt model) using the same MacBook and Thunderbolt cable - I don't have a DisplayPort cable to hand to see if that works or not so right now I can't verify if yours will work or not.

Best Answer

Related Solutions

Force external display refresh rate

IMac – Using iMac 21.5 mid 2010 as an external monitor of Macbook Pro Retina late 2013 15″

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