On Tuesday, December 23, 2014 10:18:15 AM UTC-5, Danny D. wrote:
trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:

If the ground is affecting the flame sensor, then it't the ground path
from the metal cabinet/frame of the furnace to the control board. That's
where I'd be looking. There must be a ground wire that goes from the control
board to the frame and the frame is also grounded to the incoming AC.

That's a good point in that the *control board* needs to be grounded,
as well, especially, since the *only* DC I can find is in the flame
sensor circuit.

Right now I'm looking up how to test the handful of microamps that
the flame sensor circuit needs.

I'm not sure how it works yet. I suspect the heat lowers the resistance
which then increases the current past a trip point, so, if that's the
case, I will try to measure that increased current when I figure out
how to measure the current.

AFAIK, it's a thermocouple, similar to what you'd have in a water heater.

You can probably buy a new one for not too much, might be easier than
trying to measure what u have.