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I got a somehow broken iMac Mid-2010 (Mac11,2) with a Radeon HD 4670 video card from a friend and now have to repair it.

Diagnosis history:

  1. The iMac with El Capitan installed doesn't start properly to its main volume hanging at the grey apple on light grey ground.
  2. First I discovered a wrong RAM module (PC3-8500 DDR3) though the iMac requires PC3-10600 DDR3. The iMac booted successfully to a memtest86 thumb drive. No errors are shown for either of the different RAM modules (even the PC3-8500 DDR3 module seems to work reliably). I replaced the wrong one by original Apple OEM modules.
  3. I've reinstalled different systems (Mavericks, El Capitan). None of them boots properly.
  4. All tested Recovery HDs work! (Mavericks, El Capitan) as well as Single User Mode.
  5. None of my different full OS thumb drives containing Snow Lion or Mavericks boot properly.
  6. AHT/Apple Diagnostics doesn't work.
  7. I got hands on a Apple Service Diagnostics image for this iMac and neither the EFI nor the OS part boots properly. The OS part boots to a blue screen and after a few seconds I get a white screen with some colored artifacts, the EFI part hangs at "Loading VideoController devices".

It looks like that boot volumes using a particular video card driver (apparently those installed in a full system) don't boot properly while default drivers like the one in the memtest86+ image (probably some generic Linux driver) work.

How do I continue diagnosing the probably malfunctioning graphic card?

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