On Tuesday, December 23, 2014 11:32:58 PM UTC-5, Danny D. wrote:
trader_4 wrote, on Tue, 23 Dec 2014 08:00:29 -0800:

AFAIK, it's a thermocouple, similar to what you'd have in a water
heater.
You can probably buy a new one for not too much, might be easier than
trying to measure what u have.

The flame sensor is $20, and it's on order.

At the moment, it could be any one of three things:
1. Bad flame sensor (let's hope),
2. Bad circuit board (let's hope not as the quotes are $250 & $450)
3. Bad circuit board ground (I wish).

I know you suggested I check the ground on the circuit board, but,
try as I might, I can't find a single connection "labeled" ground!
https://c4.staticflickr.com/8/7517/1...15b36c3e_b.jpg

Presumably the AC line is connected to the controller. Follow the
green wire/s. The incoming ground will be connected to the cabinet
frame, typically right where the connection is made. And typically,
you'd have a green wire at the controller, together with the hot
and neutral, that probably goes back to that incoming connection point
too.