Troubleshooting between MoBo, CPU and CPU fan

Hi folks,

I have what I think are two questions with quick yes/no answers to help me troubleshoot a HW problem:

One of my PC's suddenly stopped working whilst in use (been built and running for 18 months or more). It won't re-start, the only sign of life is that when the power button is pressed the light in on the PSU brick flashes/blinks and the CPU and case fans make a small jerking motion i.e. they start to move but only get about 2 or 3 degrees then return to their starting position, in other words there is a tiny movement but they don't actually spin.

I thought the PSU the most likely culprit but have swapped for a known-good unit which produces the same results and the old PSU has been tested and came back 'OK'.

I've pulled the motherboard out of the case and removed components, testing the power button each time, until I am left with only the MoBo, CPU and heatsink fan connected. At all stages pressing the power button produces exactly the same results as before.

So the fault seems to be either the MoBo, CPU or CPU fan.

Qn 1: I believe that a good MoBo and CPU would still boot without a heatsink fan, in which case the issue becomes binary: either MoBo or CPU. Is this right?

Qn 2: I believe a good MoBo with a faulty CPU would still have the CPU (and case) fan(s) spinning so it looks like the MoBo is the likely faulty item. Is my logic sound?

Thanks for helping me get my logic straight!

Not all MoBos would boot without a functional FAN, but it would at least beep. Depends on the MoBo and BIOS settings.

You can test the FAN without a functional MoBo by shorting the PSU to run (two pins in the 24pin connector) and pulling the 12V pin and GRND pin from the fan molex socket.

I think again that a good MoBo would beep again if nonfunctional CPU would been inserted (depends again on the MoBo).

What are the exact components involved? ...would still suspect the MoBo.

EDIT: Yes,your logic is sound :)

From what I can tell it's a faulty mobo. If it wasn't than the case fans should've still span.

Thanks folks, looks like the MoBo's going back!

FYI, it's an ASRock FM2A75M-ITX with an AMD A10 5700 and a Gelid Slim Silence sat on top.