iMac will not turn on

Hello, I hope that someone can help me since i feel as though i have exhausted every single option available online.

I have successfully upgraded my 21.5 iMac i3 3.3GHz early 2013 model with 16Gb of RAM and 1TB SSD after experiencing a very slow computer. The computer was running flawlessly with these two upgrades but after a week, the screen froze. So then i proceeded to do a hard reset by holding down the power button, then it will not power up anymore.

Two Diagnostic LEDs are lit after i hit the power and the fan is on at low speed, with absolutely no beeps or anything present on the screen.

Thinking this issue was that i must have fried the GPU since it was a hot day and i had a big external monitor hooked up for long hours, i invested in a new logic board - upgrading from a

i3 3.3GHz (820-3482-A) early 2013 model

to a refurbished

i5 2.7GHz (820-3588-A) late 2013 model

Still, no dice. I have tried putting my old RAM back into the new logic board and same results. The only critical component i havent touched is the power supply. Before i invest in a new unit, i was wondering if someone can help me diagnose and figure out my options…

Thank you all