It’s AMD Athlon 64 CPU, MSI Mobo, 512 MB DDR1 RAM based PC. It’s around 3 years old.
Have done some troubleshooting and this is what is happening.
When I Press the PowerOn button, the CPU Fan runs for approx 1 – 2 sec and then system shuts down immediately. So seems like it’s not even reaching BIOS.
I have checked PSU separately and ruled out any issues with it. Tried with another PSU and same result.
Tested the system separately outside the cabinet with only mobo, CPU with heatsink fan. Even RAM was removed. Still same behavior.
However if I unplug the CPU 12V supply and poweron, then PSU starts and is stable and so does the CPU fan runs. Since CPU power is off, no processing is happening though.