makolber wrote, on Tue, 23 Dec 2014 16:19:30 -0800:

The flame senor works by rectification.

There are only two ways to test a component:
a) (Perhaps blindly) following a test procedure, or,
b) Understanding how it works and improvising an appropriate test.

I can't blindly follow the stated test procedure because I can't
easily measure from 1 to 6 microamperes.

I can't logically improvise a reasonable test procedure because I
can't figure out how the thing works.

I did try measuring resistance from spade to tip, which varied:
0.2 ohms https://c4.staticflickr.com/8/7516/1...0760b5ca_c.jpg
0.6 ohms https://c4.staticflickr.com/8/7523/1...f1054855_c.jpg

And I verified that the flame sensor "body" is attached to ground:
0.2 ohms https://c4.staticflickr.com/8/7480/1...7e94f4f8_c.jpg

I doubled up on the ground from the flame sensor body to the chassis
ground, (it's hard to see the black alligator clip wire in the photo
below, nor the tape on the door switch, but this is that test):
https://c4.staticflickr.com/8/7534/1...93846665_b.jpg

With the flame sensor body ground doubled, I cycled the furnace,
just to make sure that a bad flame sensor ground wasn't the problem:
https://c4.staticflickr.com/8/7512/1...d60a52f2_c.jpg

In the hopes of doubling the control board ground, I looked (in vain)
everywhere I could on the controller board for a named GROUND,
but never did fine a single connection called "ground":
https://c4.staticflickr.com/8/7564/1...8288d901_c.jpg

All these tests were in vain.

I still don't know if it's a bad flame sensor or a bad circuit board.