Chosen Solution
It will not boot up completely. I have tried safe mode, apple hardware test. But get this: I removed the powersupply and checked visually and saw no burn marks. Diagnostic lights all come on. If I leave the lcd screen disconnected and connect an external monitor it boots up fine, but the fan is a lot faster, I imagine because of the temperature sensor not plugged in. What is up? I will not venture a new lcd as I have had a really bad experience on another imac in the past (brand new replacement lcd fried in 30 seconds after installing! $700 USD down the drain!). Could the mainboard be over heating but the fan going faster is compensating? Does not make sense.
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.
Hello so i have tried this on a several mac and ran into an issue with logic board that does not compensate the balance of power to the LCD, and burns it out. I’m Apple Certified and came across that it is related to logic board, once i had it replaced and the internal lcd worked as normal as well with external no speed of fans. Though in a school education working in they had the means of replacing the boards as an indiviual would suggest to save money and find another comparable in price to buy a newer model as more issue may arise in which it does in time to get that computer going longer. I have a few 2009 models and they are going strong til now 2017 but first the Hard drive needs replacement, then normally the hard drive cable and then logic board - related to power to unit or over powered and Finally LCD Issues. Thanks www.MacOSGenius.com
