Tony Hwang wrote, on Mon, 22 Dec 2014 15:58:34 -0700:

First, I'd just clean the flame sensor to get one thing out of the way.
I never ran into failed sensor but dirty one which did not work well.
Rubbing it with emery cloth always made it work again.

The flame sensor is supposed to have 50 microamps, but I
couldn't measure that low with my Fluke 75 DMM:
https://c4.staticflickr.com/8/7463/1...2757e870_b.jpg

So I removed the flame sensor to take a look at its condition:
https://c4.staticflickr.com/8/7475/1...8647f174_b.jpg

That flame sensor was in surprisingly good shape, as is everything:
https://c4.staticflickr.com/8/7517/1...71745c49_b.jpg

Even so, I steel wooled the small amount of oxide, just to be safe:
https://c4.staticflickr.com/8/7539/1...d5cf70c5_b.jpg