Danny D. wrote, on Wed, 24 Dec 2014 04:45:28 +0000:

The transformer, nominally 24VAC, tested at 28VAC, by the way.

It's not clear, to me, whether that's because of RMS versus
average measurements, but the measured 28 VAC is close enough
for our purposes to assume the switched power to the RED terminal
isn't a problem.

What's harder to understand is the unswitched COM terminal.

It seems to be an unswitched neutral, coming out of a center-tapped
transformer, especially since it doesn't seem to go to the thermostat,
but, it does go to the AC unit.

Anyway, for some purposes, I assume it's not a center-tapped
transformer, and that the COM is simply the unswitched other side
of the transformer, where the RED is the switched side of that
transformer.

But, for other purposes, I assume the COM is a "neutral" of sorts,
coming out of the middle of a 12VAC + 12VAC transformer, which makes
it a ground, of sorts.

All this probably just tells you that I don't really understand
what exactly the switched RED and unswitched COM terminals are, but,
in reality, that's not the problem with this furnace, so, that's
just a diversion away from the real problem, which is determining
whether the failed part is, either:
a) A bad flame sensor ($20 part is on order), or,
b) A bad control board ($250 to $450 part can be ordered), or,
c) A bad circuit board ground.