so I have some good news here first. I got it fixed. It was not the PSU. After several hours tinkering around on my PS4 to test the behaviour of it, I found that it shuts down when pushing a tiny little bit on the shield that covers the “B-Side” of the motherboard. I found this out during the test with the “washer-method”. You will find this method on Steve’s basic video about the PS4 On/Off problem. After it worked fine for a while with two washers on each side of the screw, I thought at first this would be the fix and thus that the cause was the APU with some loose solder joints. However, later on I found by accident that it still would shut down immediately when I pressed on that shield. I inspected the shield and found that it was slightly bent, and there was even a dent in it where it shouldn’t be. So, I assume that this caused the shield to make connection to some of the board’s components and as a result shorting out some circuits, which then caused the system to immediately cut power (thankfully just that and no blown parts…)
I then carefully tried to apply some pressure to some points of the shield and made effort to bend it back as good as I could and also removed the dent by shaping it with some metal tools. To be on the safe side, I also taped off the corresponding areas of the shield to prevent any unwanted connections.
I made another critical mistake during this repair, when I put it back together. I forgot to insert the plastic part that keeps the shield away from the bottom part of the APU. As a consequence, the PS4 also shut off immediately when I turned it on, because the shield caused shorts on the components in that area. I am lucky that nothing broke during that, please don’t make the same mistake!!
After all this it runs without any problems.
Sigh, I wish it would. I made more mistakes. I want to share them with you, so you can avoid them in case you run into similar issues / have similar ideas:
After all the testing I wanted to disassemble it one last time to put on fresh thermal paste. There was also one thermal pad missing on the RAM, so I also put some thermal paste on it. I was thinking, maybe it is a good idea to replace the 3V Lithium battery (CR 2032) on the mainboard. I took it out and replaced it. This was a mistake. I did some research on this topic and found that when the voltage of the RC circuit gets interrupted, all data on there gets erased and can only be setup again correctly and fully working, when connecting to the PSN servers. This also means that the newest firmware has to be installed on the PS4. For those of you who want to change the battery and do not care about the firmware this is not a problem, all you need to do is sync it up with the PSN. For those who would like to stay on lower firmware (in my case 6.72), this means from then on you have to use workarounds (one way is through exploits) in order to be still able to play any game, no matter if its original, fpkg or disc. Plus, the system clock will always be reset to some date around 1970 after each time powering it off. It is not the end of the world but a very annoying thing and it’ll get interesting for anyone who likes to keep using the PS4 for a long time especially when in a possible future scenario the Sony servers will not allow the PS4 to get any service. There are workarounds though and you can find lots of info about it when looking up “CMOS timebomb” or other related terms.
Well, this fix was very interesting for me as it is my first PS4, and I learned a lot about it, and actually the fix itself was super easy but the diagnosis took ages. Maybe there is someone out there who has the same issue and if so, I hope that it helps.