Vizio P42HDTV Power on Diagnosis

Question

I have a Vizio P42HDTV that won't turn on.  I believe the main and sub power boards are ok, as all the voltages check out.

How can I tell whether the turn on issue is from the power supply versus the main (DTV) board?

The unit's power light comes on when I plug it in, but pressing the power button does not cause the led to cycle to green, nor do the main power relays click.