whenever in doubt that i have a faulty hardware in my imac, the reasonable thing to do is of course run the hardware test. sure enough, it always says that everything is fine which gives me comfort(false comfort if i may add). recently, my display basically died. everything booted properly but the display just won't show. i did eventually got it running after a few tries. again, i run the hardware test, it says everything is fine. the day after, it died again so i sent it in for repair and found out it was a faulty graphic card. maybe i'm just unlucky but it seems the hardware test never shows the correct result which renders it to be useless. my friend had his logic board changed and the hardware test also missed that even though he already started experiencing problems.