iMac A1311/A1312 Graphics card
iMac Late 2009 to Late 2011 are known for their GPU failure. All models are affected to a certain extent, except the 21.5" 2009 with integrated graphics only (NVidia 9400M). All other models use a ATi/AMD Radeon HD HD4000 through HD6000 series GPU which are unreliable. The most affected is the Radeon HD6000 series in the 2011 models, then the Radeon HD5000 in the 2010 models and finally the Radeon HD4000 in the 2009 models.
There are no officially compatible graphics card available to replace them. Most cards you find on eBay or elsewhere already have been baked or will fail soon enough.
The best solution as of 2020 is to install an NVidia graphics card based on the Kepler architecture as they have native support since Mac OS X 10.7.5. However, Apple's EFI does not support the VBIOS used on the "non-Apple" cards, some people thankfully figured out how to patch the VBIOS to restore EFI video output support and native brightness adjustment on some cards.
The Core 2 machines appear not to be supported properly. For the A1311 2009 DG, it is possible to swap the logic board with the A1311 2009 IG model to get rid of the discrete graphics card. It may be possible to convert an A1311 2009 DG board to IG board by removing the parts with BOM option "MXM", and soldering the parts with BOM option "IG", but it is a lot of work. There is no known solution for the A1312 2009 Core 2 yet, the display output from the NVidia MCP is not routed at all.
If you are considering replacing the GPU in your iMac, you should REALLY read this post as it is appears to be the central repository for all things related to iMac GPU replacements - including more information on supported cards and available VBIOS: https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/
|A1312 2009 Core 2||K2100M||No||Stuck on gray screen|
|A1312 2009 Core i5/i7||K1100M||Yes||Requires PRAM reset|