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.

You're definitely on to something here!

This video says reversed polarity will cause teh flame sense to not work!
https://www.youtube.com/watch?featur...WNRqMM#t=31 6

Look at the test here for ground using AC voltage ( 2VAC):
https://www.youtube.com/watch?featur...ApSN7E#t=49 8

From this video, the FS terminal (Flame Sensor) on the board is key:
https://www.youtube.com/watch?v=JE3MzApSN7E

That control board FS terminal sends out a signal of 90VAC to the flame rod.
The flame itself, acts like a diode and a resistor, connecting the flame rod
to ground, which is the burner metal itself. Completing the circuit causes
the control board to hold open a relay, which keeps the gas flowing.

I noticed my burners were rusty on their ends, so, I will try to clean
them up. https://c4.staticflickr.com/8/7554/1...49404dd7_c.jpg

Any ideas how to clean up this burner end?