Bad PSU?

I’ve had my ai1300 for about 1.5yrs. Never an issue, and maybe not the problem. Every year in January I do a fan refresh and a good clean. Finished up, returned to desk. Powered on. Saw Asus logo. Then started booting. Just before windows the system shuts down. No big deal. Press power, fans spin up, within seconds it shuts off again. This time, no logo. Do all the usual stuff, and same issue. So now I rip into it, backing out my changes, and still same thing. I recently added a 5090 last October, removed that, same thing. Yank memory, same thing. The system does not care what I remove, including the boot drive. The MB on boot shows 40 for about 8 seconds, then 00, then turns off. I dug out my old video card and some older memory. Same deal. The CPU,memory,etc was not touched during cleanup, and I don’t use compressed air. Only the video card was removed to gain access to lower front fans. It’s an Asus x870e crosshairs hero. Coarsair 96gb, Asus 5090, 9950x3d. Never had this happen till now. Strange thing, is the giant ROG logo on the MB slowly pulsates when it shuts off. Don’t remember it doing this before. So it’s not a total loss of power, and that’s why I cannot confirm for sure if it’s the psu, motherboard,cpu. I’m picking up another psu today to rule out this part, only because it seems to be a fairly common issue with the ai1300.