I have an iMac 2012 27" with GTX680MX running 10.12.3. Recently, it suddenly operates in low resolution of 1280x720 mode, even at boot up screen. I ran Apple Hardware Test and extended test which reported no problem at the end of the diagnostic run. The screen looks normal, with no grey smudges (on white screen background). My 24" ACD when attached as secondary monitor to it, is able to display at 1920x1080 resolution. The slim iMac 2012 internal screen though is still max at 1280x720 (the only available resolution it can run) Does anyone encounter this problem before? My guess is that the iMac screen itself has turned bad and not the card or logic board itself. Any advice how I can fix it without the expensive repair? Thanks. To add on: It has the same issue when I bootcamp to Win 10. Screen resolution 1280x720. Update #1 (19th Apr 2017): Today, I hooked up my iMac to my MacBook Pro 15" 2012 (GT650M) in Target Display Mode (via TB cable). I want to know if the iMac display itself is faulty. It turns out that the iMac screen is able to display 2560x1440 nicely without any problem. Earlier, I can see the iMac is able to power the external ACD in 1920 x 1080 mode. This shows that the GTX680MX is still able to power the screen to at least HD 1920x1080 mode. However, on its own internal iMac display, only 1280x720 resolution is available. So it is not the display problem, it is not the graphics card problem, what could be the cause that my iMac can only run on 1280x720?