The PSU provides a permanent +5 VDC on the side connector. This same connector has an input, used to tell the PSU to provide +12 VDC on the 2 big metallic legs soldered on the motherboard.
To make basic tests on the PSU you can:
- remove it from the console
- plug it in the AC with the power cord (from this point, manipulate the PSU only by touching plastic parts!)
- connect a multimeter on the side connector and check that you get +5VDC on some pins (can't remember which ones, just test)
- verify that you have 0 Volt on the 2 holes where the big metallic legs are supposed to be plugged in
- use a wire to shortcut the first and the last pins of the PSU side connector, this should tell the PSU to provide +12VDC
- check with the multimeter that you get those +12VDC on the 2 holes
If yes, your PSU seems to be working.
If +5VDC and/or +12VDC is missing, the PSU is the culprit.
You can find tutorials on the net, just search "PS4 ADP-240 PSU Test" for instance.