4,1 Mac Pro, won't power on

[verdejt]

Start with the basics, then go from there. Do you have a copy of the service manual? From Page 34 of the service manual.

 

[flehman]

I have no reason to believe the CPU is the issue. It looks in good condition. And the backlight issue progressively became worse over almost a full year. I'm sure if the CPU was damaged, the computer would instantly become unusable.

CPU could look fine from the outside but still be cooked given the events you described in the first post. A replacement CPU is cheap if you buy a slower one like a 2.26, just to have a known working part to switch in. In the absence of good diagnostic data, about the only option I can see is to start switching out components, starting with the cheapest stuff first.