makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:

The flame sensor probe acts as a tiny battery and the
controller needs to sense that current. The flame sensor
current is relative to the furnce ground which needs to
be connectd to the AC ground and to the controller ground.

You're the first person to describe *how* the flame sensor
works, so, I appreciate that input, as the first step of
measuring something is knowing how it works.

I had suspected that the DC current through the flame sensor
changes with heat, such that the controller senses that change.

In the same vein, that's the same as saying the resistance
changes, such that the voltage drop across the flame sensor
and ground changes (which, I think, is your battery analogy).

I guess I could remove the flame sensor and put a bunsen
burner on it and measure the resistance change, but, right
now I'm looking up for how to test the current at such low
microamperes, when my Fluke 75 only has a 300ma scale.

I do have a cheap Radio-Shack emergency open-jaw ammeter in
my car trunk, so, I'll dig that up, but, I doubt it's
sensitive enough to go down to a microampere.

I guess also make sure the burner jets are also grounded
to the chassis. The flame is grounded to the burner
jets actually.

The flame is grounded? (I don't understand that statement.)