I discovered that a blown L9000 is my problem as well on a Mid-2010 iMac 21.5inch. Attached is an image of the blown inductor, for reference to others.
You do have to remove the GPU from the slot and peel back the insulating tape to see the damage. One clue for me was you could see the tape was burnt thru, even before peeling it back to reveal the damaged L9000.
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