Original post by: Charles ,
I was able to run AHT test with external monitor plugged in: I get the error4SNS/1/C0000008: TL0p - - 124. Related to the temperature sensor on LCD panel, which is odd because when I am doing the testing the lcd panel is not plugged in. That leaves me with the actual sensor on the mainboard. I have looked up my history on ifixit and this is the exact error I received when I wasted a brand new lcd panel on another iMac a couple of years ago. I think we have our answer: mainboard needs replacement because of bad temp sensor. I will recommend not replacing the mainboard as this is a late 2009 model, and the prices to get one in Canadian dollars is outrageous.