Chosen Solution
I have a late 2009 model IMac 27”, Checking the Diagnostic LEDs only LED 1 is on. Testing the power switch works fine Testing Logic Board power, the incoming voltage on pin 4 12v Pin 12 starts at 4v then goes down to 0v after power button has been pressed Pin 11 then shows 12v yet no led 2 or any fans/hdd activity. Guessing I have a bad connection from moving around the motherboard but if anyone else has anything else to try? Update 27/05/19: After looking at it without being so tired The graphics card was shorting on the heat sink The system boots without and issue if the heatsink is removed from the graphics card I am in the process of getting another heatsink will update if successful. (One that is intended for the graphics card installed)
Do you use a good surge suppressor or have a UPS which your system and it’s peripherals are all plugged into? I suspecting you had a power spike or a lightning hit nearby which has zapped you. The power supply needs replacing for starters, there’s a good chance the logic board is damaged too. iMac Intel 27" EMC 2309, 2374, 2390 or 2429 Power Supply and here’s the iFixit guide: iMac Intel 27" EMC 2309 and 2374 Power Supply Replacement
