Screen resoluton stuck on 27 imac

Hello SOAR1,

Thanks for using Apple Support Communities.

If I understand your post correctly you are unable to see any other screen resolution options in the Displays System Preferences, even after reinstalling Mac OS X and resetting your NVRAM. To get started with this issue I'd like you to select the option to "Detect Displays" in your Displays System Preferences. In order for this option to appear please follow through the steps below:

1. Click on the Apple logo in the menu bar
2. Open System Preferences
3. While holding down the Option key on the keyboard open Displays (keep Option held down)
4. With Option held down you should see an option for Detect Displays
5. Click on Detect Displays

OS X El Capitan: If your display doesn’t look right

If the issue persists after following the above directions, please test this issue while in safe mode:

1. Choose Apple menu > Shut Down.

2. After your Mac shuts down, wait 10 seconds, then press the power button.

3. Immediately after you hear the startup tone, hold down the Shift key.

   You should press the Shift key as soon as possible after you hear the startup tone, but not before.

4. Release the Shift key when you see the gray Apple logo and progress indicator.

To leave safe mode, restart your Mac again, but don’t hold down any keys during startup.

OS X El Capitan: Start up in safe mode

Take care.

Hi SOAR1,

What about safe mode? That will help us determine whether there could be a 3rd party application preventing the resolutions from appearing.

Cheers.

Hey there SOAR1,

Next I'd like you to test this by creating a new user on your Mac, and testing if the resolution also fails to appear correctly.

Create a test user account

1. From the Apple menu, choose System Preferences.
2. Click the Users & Groups icon in the System Preferences window.
3. Click the lock icon, then enter an administrator name and password.
4. Click the Add button (+) below the list of users.
5. Choose a type of user from the New Account pop-up menu.
6. Give the user a full name, account name, and password.
7. Click OK.
8. Close the System Preferences window.

If there are documents from your original user account that you want to test with, place a copy of these items in the Shared folder in the Users folder. Press the Option key while dragging a file to this folder to create a copy.

Log in to the test user account

Log out of your current user account by choosing Log Out from the Apple menu, then log in to the new account you created. If you're prompted to sign in with your iCloud account or Apple ID when you log in, skip this step.

Try reproducing the issue

How to test an issue in another user account on your Mac

If the issue persists, then it's likely a hardware fault, and the Mac would need to be serviced. You can learn more about getting your Mac serviced by looking at the information in the link below.

Service Answer Center - Mac desktops

Take care.