After doing some analysis, i think i know why it's shutting down, i'm just not sure how to fix it. I suspected my system...

FAN IS WORKING!! Ok, I took the PSU out - 2.5mm hex didn't work for screws, I used a 3/32 with ball end to remove screws. I think I know why there is no mention of PSU fan in the iFixit manual, because it is such a hassel. It is attached to a piece of metal which cannot be removed (see pic1.jpg). It's attached by some rubber fasteners (see pic2.jpg). Once I loosened fan, I realized the wire goes behind the board, and routes to front near the PCIe Fan (see pic3.jpg). In order to disconnect this cable, I had to remove the PCIe fan. After I disconnected PSU Fan cable, I used my datavac duster (http://a.co/bJ0zQUz) - very powerful, on all components (PSU, PSU FAN connectors on wire and board), and went ahead and did the entire computer (although it was already clean). I plugged everything back in, said a prayer, and booted it up. THE FAN HAD A CONSTANT SPIN. Afterwards, I installed the fan software (https://www.crystalidea.com/macs-fan-control/download) and it was able to read the RPMs (see pic4.jpg) and it...

missing a very crucial step before step 6. if you don't remove the cover (where cd rom is) and unplug the PSU cable, you'll damage something.

for screws, 2.5mm didn't work for me, but i used a 3/32 hex with ball end, and at an angle, it will loosen screws.