The same thought has occurred to me. If the power to the CPU fan is somehow triggered by the CPU, then that may be where the problem lies. Impossible for me to test, though.

Unless you want to deliver it back to those guys, I might have a couple of things to try;

1. Alternate between GPU and onboard output(s) (pc<>screen)

2. Try taking out the battery / pin, to reset cmos. It sounds to me that everything gets power and will spin up, but possibly the BIOS/CPU hangs...

3. Some computers don't like being moved to much (only experienced on older pc's), or assembled wrong, might try disassemble and assemble the pc, just to be sure there's no lose ends.

Sorry, but as you said, it's a shame that your son can't use the computer, most likely because they didn't check it for faulty component/assemble... :(

Either way - good luck! And merry Christmas!
Next year you'll build it together, right? :)

I have very similar MB to yours (Gigabyte-870A-USB3) and my fan was not starting at all on startup at first and I thought I did something wrong or it's faulty. I had to change its settings in BIOS to make it run on boot, so it's not necessarily the fan.

There is no onboard GPU, hence, no output either.
Yeah, I could give this a try.
I already did, more or less.
Hehe, it's a brand new desktop machine. You think he's getting another one in a year? :-D

Yes, that's true...usually, but sometimes something happens and all obvious methods do not work. Some time ago my friend called me that he gets nothing besides a black screen when he powers ON his computer. I went to him and tried every ususal method in the book. I reset the BIOS - nothing, I removed both RAM stick - no beeping, I unpluged evrything (drives, HDD, etc.) - nothing, The computer had power, everything was working but there was nothing on the screen. So the verdict was obvious, something is broken, propably MB. So I removed the graphics card, powerd ON the computer and guess what? It started beeping that there is no RAM inserted. I turned the computer OFF, inserted RAM back to it's place, inserted the graphics card back in and pluged it back to the PSU, then I powerd the computer ON again and it worked again. I saw this trick years ago and it seems that sometimes something happens that the GPU is blocking the whole system. So I still think it's worth a try.

Here's the conclusion to the story.

I tried a few more things, but nothing I did made any difference. The one thing I didn't do was to check the CPU. This was intentional, because I reasoned that if the problem was that they had bent or broken one of the pins, I didn't want them to be able to claim that I had done it.

In the end I had to take it to the shop I ordered it from. This involved a trip of an hour by two buses and a train, carrying a midi tower desktop PC. I had called them beforehand, and they claimed that naturally they had tested it, and that they wouldn't have sent it otherwise, and that the problem was probably caused by mishandling during shipping.

When we got there (I brought my son with me to invoke sympathy, since it was his Christmas present), the service technician behind the counter took the machine and said he'd look at it "later". I asked for clarification and explained how difficult it was for me to travel between the shop and my home, with my son providing backup in the form of a wobbly lower lip ;-) The guy then promised to look at it within the next two hours, and he created a support ticket with my phone number on it. My son and I then went to look at shops and get something to eat in order to pass the time.

When we returned two hours later, he still hadn't looked at it. I got quite angry at this point, since he had my number, and could easily have called to say he wouldn't have time for it after all. Still, he promised to have it ready the next day.

So, we had to go back home and return the day after. Fortunately he'd kept his promise, and the machine was ready to go. The official reason printed on the support ticket receipt? The CPU had been mounted incorrectly. This pissed me off for two reasons. Firstly because I could easily have corrected it myself if I had known (although I still don't understand how you can mount a CPU incorrectly), and secondly because it proved without a doubt that the guy I spoke to on the phone was lying when he said that they had tested the machine before shipping it.

At least now the PC works, and my son is happy.

tl;dr - The CPU was mounted incorrectly.