iMac: grey screen, vertical green lines, not booting

[atlanticza]

My 27" iMac suddenly died reading a web page as if there was a power cut. On trying to reboot, one is faced with the typical grey screen and progress bar along with a number of quite broad green vertical lines. Progress stops, then blank white screen appears, then a reboot back to grey screen and green bars... And again. I've googled a lot but with no definitive answer... Logic board? Graphics card issue? Some help would be appreciated.

 

[ApfelKuchen]

My 27" iMac suddenly died reading a web page as if there was a power cut. On trying to reboot, one is faced with the typical grey screen and progress bar along with a number of quite broad green vertical lines. Progress stops, then blank white screen appears, then a reboot back to grey screen and green bars... And again. I've googled a lot but with no definitive answer... Logic board? Graphics card issue? Some help would be appreciated.

It's too early for a "definitive answer." You need to begin with basic, broad troubleshooting steps.

You said, "not booting," but what's happening by your description is that it's failing during boot (OS X launch)

Have you tried Safe Mode: https://support.apple.com/kb/PH18760

If that fails: booting to Recovery or Internet Recovery, and run Disk Utility to Verify Disk? https://support.apple.com/en-us/HT201314

Have you tried to Reset PRAM? https://support.apple.com/kb/PH18761
There's more, but this is a start. Any of these might resolve the issue, or if not, help narrow the possible causes.

 

[atlanticza]

Thanks for the advice. And yes I have tried resetting PRAM and Recovery but to no effect. Screen just hangs on white screen.

 

[Taz Mangus]

Thanks for the advice. And yes I have tried resetting PRAM and Recovery but to no effect. Screen just hangs on white screen.

You may need to have your computer looked at by Apple so that they run diagnostics on it.

 

[atlanticza]

Fixed!

I took the advice and handed the iMac in.... and yes it is a faulty graphics card which will be fixed free of charge.

iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program
Apple has determined that some AMD Radeon HD 6970M video cards used in 27-inch iMac computers with 3.1GHz quad-core Intel Core i5 or 3.4GHz quad-core Intel Core i7 processors may fail, causing the computer’s display to appear distorted, white or blue with vertical lines, or to turn black. iMac computers with affected video cards were sold between May 2011 and October 2012.

If the iMac (27-inch) meets these requirements, Apple will replace the video card free of charge for four years after the first retail sale of the computer.