27" iMac doesn't boot past white screen, not-responsive

"Hi,

I had the same problem on my iMac dual-core 3.06gHz. Stupidly I put in a firmware password and forgot it and I had recently blocked anybody accessing my CD drives etc. Even I was blocked. Therefore I couldn't wipe the hard drive and reload OSX. I got the blank white screen.

This method got my iMac back. To do this you need to alter your RAM configuration first by adding or removing memory. This forces the PRAM to take action. You will still need enough RAM to run the machine of course.

1) After changing your RAM either up or down.

2) Using a wired keyboard, start computer with keys COMMAND-OPTION-P-R pressed in.

You MUST use the COMMAND-OPTION keys on the RIGHT side of the keyboard.

3) Let the machine boot THREE times (mouse lights up on each boot cycle, turn it over to see)

4) After the THIRD or FOURTH boot cycle hold the OPTION key down ONLY.

5) A BOOT MENU should appear. I cannot remember if I hit EJECT on the boot menu or on the keyboard. Try each if it doesn't work first time.

You can access your CD drive now, put in your BOOT CD (OSX disk).

Reboot into Single User mode and reload your OSX system.

At step (5) You may not have to reload your system files, just set up your PRAM."

Same problem only my solution was to press on D while holding the power button until apple logo came up then the bar showing it was loading something. Afterward it worked fine.

"Can you boot from an external HD or flash drive?

If you can you will likely need to run Apples' Disk Utility to straighten things up. I would strongly recommend clearing out some of the stuff you have on your disk as you likely ran out of space and/or have a highly fragmented HD. Try using Drive Genius or other good defragmenter app to defrag the HD.

If you can't boot with an external drive then your problem is not HD/OS related.

But given the fact you can get the system to run and access the HD in single user mode I suspect its the HD that just needs attention."

"Thx for writing this article - had the same problems. And actually repaired earliere iMacs 27. So have a bit of insight in the tech. But was getting grey hairs over this one, and wen't through all possible scenarios (as you) then after two days found this o/ :)

Thanks again, will contact Apple tomorrow."

"i have got same issue and the problem is Graphic card. The solution have simple if u have a little electronic skills. teardown imac and remove the graphic card. and also remove gpu coller. your graphic card must be like this picture of gpu after that u must be very carefull this proses. first u clean gently with alcohol and toothbrush. after that u bake in oven like this video after that your graphic card work.

this solution is riskly and get your own risk.

your gpu has died also, maybe u can take it life back again"