Question : Problem: No video.  No POST.

I'm attempting to diagnose why there is no video display on a T6420 Emachine, running Media Center OS.  The CPU is an Athlon 64 3400+ and the memory consists of two sticks of 512 Mb DDR2 SDRAM.  Video is entegrated on the MB.  I've done the following:  (a)  Installled new PCI-E video card.  No change.  (b) Removed modem & disconnected HHD & Optical drive.  No change.  (c)  Tested power supply. Showed to be okay.  (d)  Replaced power supply.  No change.  (e)  Inserted PCI POST diagnostics card.  All circuits are lit, as though they are functioning, but no codes whatsoever show on the display.  (f)  CPU fan & case fan running well.  (g)  Changed to a known working monitor.  No change.  Any suggestions?

Answer : Problem: No video.  No POST.

I'm leaning towards CPU or motherboard. I would be guessing motherboard first. See if you can beg/borrow/buy one and try changing it out. If it's not the motherboard, then it's very likely the CPU. Just to be safe, you could try running it with only one stick of ram just to see if it's different, but I doubt it will be.