I had a problem with my HP desktop. When I plug in the AC cord, I already hear beeps and red light flashing. And when I tried to power it on, nothing happened (no fan turning) so I believed it was a power supply issue. But when I replaced with a new PSU, same error occurred.
Now I disconnected almost all connectors on my motherboard. What is left connected were wires from new PSU, internal speaker (to hear the beeps), processor heat sink fan, memory RAM and power button/LED connector. But same result.. 4 beeps and red light flashing... Even without pressing the power on button!!