Chosen Solution
I have 27" iMac 27 i7 2009 (logic board 820-2733-A) It’s working sometimes and sometimes not. But only when I unplug power cord. After few seconds I plug the cord back in to start iMac its start normally but sometimes this trick is not working. I already replaced the power supply, but same issue. I also replaced CMOS cell. When I plug power cord the first diagnostic LED lights. Not sure is this power supply fault or logic board I’m confused! plz help. sry for my bad english
It sounds like your SMC services are getting in the way of allowing you to start the system. So when you disconnect the AC line cord you’re allowing SMC to reset which then lets the system start but SMC then detects a problem and put the system into fail safe mode. Depending on how long you wait the capacitors on the logic board have enough charge to keep it in the locked state. Here’s a good T/N which explains things: How to reset the System Management Controller (SMC) on your Mac. So why is SMC stopping me? Given the age of your system I would suspect you have a cooling issue. This could be as simple as dust buildup or a fan needs replacing (and a few other possibilities). I would recommend getting a good thermal monitoring app like: TG Pro depending on your OS you may need an older version. I would start their, paste a screen capture of the apps main screen here so we can see whats happening.
