Oren wrote, on Wed, 31 Dec 2014 15:46:19 -0800:

My guess is the metal expands and opens a crack in the ceramic (not
visible). I've read about this on a gas grill and stove igniters.
Don't recall the details - sorry. I was hinting about it earlier in
the thread. Would that be a short to ground?

I just noticed this, so sorry for not responding sooner.
I'm still amazed that the heat sensor was bad, simply because there
was almost nothing to it.

I guess if, under heat, a crack allows electrons to "jump" to ground,
both ways, then the control board wouldn't measure any DC current
(because it would be AC). I'm guessing though.