Answer to "How come my iMac keeps shutting down after these tries to fix?"LED 4 indicates that the iMac is detecting over temperature conditions That is not correct on this model of iMac: LED #1 (left-most) Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power source is disconnected or the power supply is faulty. LED #2 Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly. LED #3 Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement. LED...
Answer to "Can I upgrade the graphics card?"The new guide here shows how to replace the video card in this iMac.
Answer to "Change Graphic Cards on this Mac?"As I was troubled by the conflicting information on this page/site, I wanted to testify I did just complete replacing the video card in my A1225 / EMC2267 24" iMac. I bought the Mac new from Apple in 2009 with the ATI Radeon 4850 HD video card upgrade. After 4 years, the VRAM died...the picture would scramble at the logon screen unless I booted into safe mode. In safe mode, screen covered in blue dots, TechTool Pro confirmed bad VRAM. I purchased a new card from Impact Computers in Florida. I had to fully remove the logic board to get at the video card, but with only three more screws it pulled free from the MXM connector, heat sink and all. I've got pictures of the whole process if anyone's interested. I'm confident one could upgrade the nVidia GT120 or 130 mid-line machines to the ATI card the same way. It may be the baseline machines (9400M graphics) don't have the MXM socket installed on the motherboard and are non-upgradeable.