Danny D. wrote, on Wed, 24 Dec 2014 05:11:13 +0000:

a) A bad flame sensor ($20 part is on order), or,
b) A bad control board ($250 to $450 part can be ordered), or,
c) A bad circuit board ground.

Now that I've seen the insides of a flame-sense rod, I can't imagine,
other than getting dirty, how they can *not* work.

It's just a steel rod inside a ceramic insulator.

Nothing more than that. It's about as simple a construction as can
be imagined.

In fact, this guy, who says he's an instructor, says he can't get
a flame rod to fail!
https://www.youtube.com/watch?featur...8450mM#t=21 7

He says you can clean it with just about anything (even a file):
https://www.youtube.com/watch?featur...8450mM#t=39 0

Tomorrow I'm going to three more aggressive ways to clean it:
https://www.youtube.com/watch?v=6DsKn1Rw9Yo

But, it's looking less and less like the flame rod went bad.