So, PC was on for about 4 hours one day, I came in at lunch and saw the GFX card fan was on full, no display.
I have since replaced motherboard, CPU, Memory & GFX card all for new as a system bundle so all components should be compatible. Breadboarded the motherboard as well and checked both ATX and CPU power plugged in. Case fan and CPU fan spins. still no clues!
Tested 600W PSU CPU 8 pin and ATX cable using multi-meter- all voltages were within tolerance, apart from the GREY power cable which should according to an article I read jump to 5V when there is load on the PSU - it essentially is needed to power the CPU so that it posts. This is outputting nothing, and after unplugging the CPU fan to see if the heatsink got warm after maybe 5-10 minutes, also couldnt feel any heat coming from the heatsink. makes me think the CPU might not be doing anything as they normally get hot prity fast even when not under load.
Is this a faulty grey cable on the ATX PSU feed ? as I say, I have replaced everything for new apart from PSU which is OCZ and only about 3.5 years old.
could really do with some help please from an expert as all I can think of now is to replace PSU. Seem to have tried everything else. I have not re-seated memory or cleared CMOS on the new system bundle because I got the same result on my old PC with different components and I had already tried this anyway.