On Tuesday, December 23, 2014 7:19:34 PM UTC-5, wrote:
Ok, im a little wrong.
The flame senor works by rectification. The controller applies a small AC voltage and the flame lets the current flow in one direction only in effect creating a small DC current. The battery analogy is not exactly right.
Goggle flame rectification sensor.

Mark

Well, you learn something new all the time. I thought the flame sensor
was a thermocouple. But from some googling, I see you're right. I never
even knew this phenomenon existed. It apparently works by using the
flame itself to conduct a small current. And somehow the AC gets half-waved
rectified in the process.

One big difference with testing, would appear to be that the rod type
sensor won't generate any voltage from just heating it. It has to have
an AC current applied, so the current test would have to be done with
it installed, connected to the controller.

Now the advice to clean them makes sense. If it was a thermocouple,
whether it had some dirt on it or not should not affect the operation.