Power Supply or Systemboard?

A few days ago, a friend of mine asked me to fix his PC that would not turn on. I asked him all of the usual questions: (as I was thinking in my head is it going to be the power supply or systemboard?)

When did this start? Last night when I was trying to turn it on.
What happened before this? The computer just shut off by itself.
Has it ever done this before? No

So from there, I began thinking in my head on at least 3 reasons why a computer would not turn back on:
1) The power supply unit (PSU) is dead.
2) The systemboard is dead.
3) The CPU is dead.

Based on 10 years of experience, these are the top 3 reasons that a computer would not turn back on after pressing the power button. I agreed to take the computer home and begin troubleshooting. First thing I did was clean the inside of the PC since it was full of dust. I have had other instances where a PC did not power on and after cleaning out the dust, it was perfectly fine again. Next, I tried turning on the computer and obviously, it did not power on....no fans running....nothing. BUT, the green light on the power supply was working as well as the little LED light on the systemboard. Although I initially thought that the PSU was infact dead, I started doubting myself after reading some online troubleshooting about this type of issue.

Sidenote: Always go with your gut instinct....I am learning that this is true for multiple things in life. 

So since at this point thinking that the power supply is dead, I tell the client that either the systemboard or CPU are most likely the issue. I advised them that unless we replace both at once, it is difficult to tell which one might be bad. I took a chance thinking it was the systemboard so I purchased a replacement one online and tested it. Guess what.....SAME ISSUE!!! The computer still did not turn on! 

I then remembered that I had a spare PC in my closet that I do not use often and I figured that I could use the power supply from it to test with.....sure enough, the PC booted up like normal. But all of the online troubleshooting I did lead me to believe that the power supply was bad......

So I decided to test the PSU offline with the paperclip method. (Please adhere to the caution warnings when trying this). The PSU worked! But it was obviously still bad because it was not powering on the computer. It may have been generating minimal power due to the paperclip but that's it. 

Conclusion......after ordering a replacement power supply, the computer is back up and running. I was able to return the systemboard since it was not the issue. Again, it is tricky to troubleshoot power issues when some lights come on or fans work, etc. The best way to be prepared is to have a spare part to test with. And again, go with your gut instinct! If I would have tested my spare power supply first, it would have saved me time and money.