Hi to all,
Experienced exactly the same problem one month 1/2 ago.
I did a lot of things, hoping to solve the issue : checking the RAM, changing several times the logic board battery, cleaning the fan, checking the graphic cards, cleaning the CPU and heaters, even changing the thermal paste, changing the PSU...
Well, a lot...
Nothing helped, it won't definitely start up.
The only way I found to work again has been to buy a new one on the Bay, not wanting to change the logic board, as the whole price of those changes would raise at the level of a new one.
Today, I still think the issue came from the logic board, so I put it aside, and one day...
Good luck to all !