🔌 Old desktop won't boot, trying to figure out if it's the motherboard or the power supply

My desktop, close to six years old now, suddenly refused to boot up last week, no display output at all, though the case fans do spin briefly and then stop after just a couple of seconds each time I actually try powering it on. Genuinely not sure whether this points toward a dead power supply, a failing motherboard, or possibly something else entirely that I have not properly considered yet given my fairly limited hands on hardware troubleshooting experience so far. Tried the basic steps I could find online myself, reseating the RAM sticks individually, checking that all the actual power cables were properly and firmly connected, even trying a different wall outlet entirely in case the actual issue was something as simple as a faulty socket rather than the computer itself. None of that changed the specific behavior at all, same brief fan spin followed immediately by complete silence and no display each single time. A friend who works with computers casually as a hobby suggested it could genuinely be either component honestly, and that without an actual spare power supply or motherboard specifically to swap in for testing, it is genuinely difficult to properly diagnose which one is actually the real culprit here without spending money on a part that might not even end up being the specific correct fix needed. Took it to a small local repair shop today just to get an actual opinion, and the technician there thinks it is most likely the power supply based on the specific fan behavior alone, though he was honest that he could not be fully certain without actually opening it up and testing it directly himself with proper equipment. Getting a replacement power supply is obviously the cheaper option of the two components to actually test first, so I am leaning toward starting there before considering anything more expensive like the motherboard itself, which would probably mean a much bigger overall repair cost if that ends up being the actual real problem instead. Has anyone dealt with this exact specific symptom before themselves, brief fan spin then complete silence, and did it usually turn out to be the power supply, or was the motherboard actually the real culprit more often in your own personal experience with something like this?

Tried booting with just a single RAM stick installed at a time before assuming it was the power supply or motherboard, actually turned out to be a single bad RAM stick causing that exact same fan spin and silence pattern for me once, cheap and quick test worth ruling out first since it costs nothing beyond a few extra minutes of your own time.

Had the identical brief fan spin followed by total silence on my own six year old desktop last year, turned out to be the power supply exactly like your technician suspects, swapped it out myself with a basic replacement and the machine booted up normally on the very first try afterward. Definitely start with the cheaper component first like you are already planning.