iMac 21.5" 2015 A1418 power problem

Hello guys, got a iMac 21.5" late 2015 A1418 which does not turn on, no power. Opened it up and plugged it in good outlet. Then, Logic board LED light #1 and #2 flashes green two or three times and turns off. After these flashes (and off), no LED lights will come on whatsoever, unplug and plug, press power button, etc.... nothing else will happen, no green LED whatsoever.

So I thought the power supply unit is bad. Ordered one and install it correctly. After installation, first time I plugged it in, the logic board LED light #1 and #2 flashes green, two or three times, and turns off. After these, same as original power supply, nothing will happen.

So I returned the power supply and put original power supply back in and left it for a week. Then when I try it again, same thing happens, LED light #1 and #2 flashes and turns off, then nothing will happen.

I can not find the answer from Apple diagnostic LED for such symptems.

Is it (1) power supply unit, or (2) logic board gone bad?