Question HP Compaq Elite 8300 Turns on then turns off right away. Blank screen, No POST, Green led on, Fan spins

Sep 20, 2022
Hi all
I have a HP Compaq 8300 mobo I got from ebay. But I have no display or POST, green led, CPU Fan spins and shuts down right away.
CPU is an i7. The PSU is a 6+4 connector and not 20 pin
Have done all the usual troubleshooting:
  • all cables connected, aux led is green, no blinking
  • when I press the power button the power led turns green (no blinking) and the fans starts
  • no beep, no red led, no blinking, but nothing happens on the display (connected via VGA)
  • after a few seconds, it shuts down
I tried the following:
  • remove dust with a vacuum cleaner
  • reseat ram
  • leave just one memory stick
  • reseat CPU
  • reset CMOS
  • replace the CMOS battery
  • disconnect everything non-essential
  • remove all ram sticks, tried different combinations
-tested the power supply with a PSU meter.

So now, I'm stumped and frustrated because I've been working on this way too long. HELP!!