imac27 late2013 problem stuck on 1280x720

Hi oamkham,

I understand that you are experiencing a display resolution issue, and have already tried several relevant troubleshooting steps. There is one more step I would like for you to try:

Reset the resolution

Start by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:

1. Start up in Safe Mode.
2. From the Apple () menu, choose System Preferences.
3. Choose Displays from the View menu to open the preferences pane.
4. Select any resolution and refresh rate that your display supports.
5. Restart your computer.

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support

https://support.apple.com/en-us/HT201177

It might also be helpful to go ahead and reset the PRAM on your iMac again just for the sake of being thorough. Hope this helps. Thanks for coming to the Apple Support Communities!

Regards,

Braden

Hi again oamkham,

Thank you for using the Apple Support Community. I would like to sugges that you gain more information about this issue by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action.

Cheers,

Braden

Did you manage to resolve the issue?

one of my macs identical to your imac has this issue. If I were betting person sounds like driver problem.

Can someone whose issue was solved reply please? I have been facing the exact same issue the past few days and have not been able to get much help browsing the available info.