Question New build powers on for a second then turns off. Could it be CPU MoBo Incompatibility?

Jun 3, 2020
Hey, guys. Hoping for a little advice...

I've done about a dozen or so successful builds, so I'm not a total novice, but today I ran into a problem I've never encountered.

After getting my new build assembled, I pushed the power button and it powered on for a second or two then turned off. I did some searching and found a few suggestions. I tried resetting the CMOS, checked all of my connections (yes, I remembered the 4 pin power), reseated the ram... nothing helped.
All of the components are brand new except for the motherboard. It's an ASUS Prime Z370-A. The CPU is an i5-9400. I now know that some Z370-A motherboards require a BIOS update to work with 9th gen CPUs , so I suspected that might be the problem, but would that cause the symptoms that I am seeing? I don't know what happens when you try to combine a CPU with an incompatible motherboard. Would it cause this power issue? From what I have read, it usually results in a black screen, not a loss of power, but I couldn't find a whole lot on the subject.

Basically, I'm trying to figure out how likely it is that MoBo/CPU incompatibility is the issue. If it's pretty likely, then I'll get an 8th gen CPU and try that, but I don't want to go to that trouble and expense if the power issue I'm having isn't a symptom of that incompatibility.

If anyone has experience with this sort of thing and can tell me what I'm dealing with, I would GREATLY appreciate it. Thanks!!!!!!

ASUS Prime Z370-A
MSI GTX 1660 Ventus SX
Geil Super Luce 3200mhz RGB RAM
500 watt EVGA PSU
Team Group 256gb M.2 SSD
Phanteks Metallic Gear Neo Air Case