Early 2015 MacBook Pro 13 inch, replaced motherboard but still will not power up!

I have an Early 2015 MacBook Pro 13 inch. It will not power up with or without the battery connected. The MagSafe connector turns green then amber whether or not the battery is connected. I tried with two different chargers and two different DC-in power boards. So I figured it was an issue with the motherboard. I replaced the motherboard with the 3.1GHz Core-i7 motherboard (instead of the original 2.7GHz Core-i5, both are MacBook Pro12,1 motherboards) and it still behaves the same way!

An SMC reset does cause the amber light to turn green for a moment, then amber again, so the keyboard communicates with either motherboard fine.

It must be something on the top case that's causing both motherboards not to power on? I did disconnect the display and still nothing, unless the fan doesn't normally start spinning at initial power on? With a known good MagSafe charger connected for an hour, it still glows amber, and everything is cold including the MagSafe connector itself!

Any ideas? Thanks!