Discussion PC turns on but doesn't post, shuts down after 30-40 seconds

Feb 19, 2019
Hello everyone !
My friend has a PC that randomly died the other day. When the power button is pressed the PC turns on but doesn't post and then shuts down after 30 to 40 seconds. (Here I would like to say that it DOES NOT bootloop or restart, just shuts down completely)
It doesn't write anything on a display.
We initially thought it was overheating cause another friend of mine had the same problem, the difference is that his PC did post before it shut down but this one does not. Everything seems to be working, the PSU fan spins, hard drive spins, graphics card fan spins, CPU fan spins and the front panel LEDs turn on.
The PC has no buzzer so we can not tell if it feeds back an error code but I don't think there is a problem with any hardware being installed improperly since it was working the day prior and was also untouched.

Things that we tested and/or seem to be working properly :
• PSU - Gives power to the whole system
• Graphics Card - Fan spins, system also doesn't post without it
• RAM - Swapped every DIMM in every slot
• Motherboard - Gives power to everything (Drives, PCIe, fans, LEDs)
• Overheating - Fresh thermal paste installed with proper cleaning

Things that we didn't test :
• Error codes from a buzzer (one is not included)
• CPU swap
• PSU swap (since it does give power)

Based on all these, what do you think is at fault ? Personally I think the CPU kicked the bucket cause it was never cleaned and was allowed to overheat with old dried out thermal paste on it but it could be something entirely different, so I want your opinions to isolate the problem down to what could be causing the system to not post. All opinions are welcome. Thanks for your time !
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