On Wed, 31 Dec 2014 05:22:55 +0000 (UTC), "Danny D."
wrote:

Danny D. wrote, on Fri, 19 Dec 2014 23:56:58 +0000:

It just started getting cold in California, so, the heater power
was turned on, and ... and ... and ... uh ... um ... no heat.
https://c4.staticflickr.com/8/7508/1...4311f464_k.jpg

Update!

The heater certainly hasn't been turned on in a year, and maybe
in two years (as last year was warm). I don't remember, but, anyway,
the problem first looked like a reverse wire but the code was
basically constantly blinking.

As far as I can tell, that just means whatever old code was in
there has been wiped from the memory, so, it just blinked
incessantly to tell us that.

Anyway, we narrowed the problem to the fact that the flame wasn't
sensed, and, that meant only one of three things:
1. Bad flame sensor
2. Bad ground for the flame sensor circuit (through the flame)
3. Bad control board sensing of the flame sensor circuit.

I cleaned the flame sensor to no avail.
Replacing the $20 flame sensor solved the problem!

It's amazing that a simple stainless steel rod could fail, but,
it apparently did! Who knows why or how.

The way the flame sensor works is that 90VAC (nominal) is sent
to the flame sensor, which is just a steel rod with a single
electrical connection. When there is a flame, electrons flow
from the steel rod to the chassis ground of the burners through
the flame (rectifying the 90VAC to DC). The flame sensor circuit
on the control board senses the 5ua of DC current, and keeps the
relays on which control the propane gas flow.

The moment the 5ua of current isn't sensed, the control board
shuts down the relays controlling the flame, which is what
was happening. So, there was nothing wrong with the furnace,
other than the flame wasn't being sensed.

With the new flame sensor, everything is working fine.
https://c1.staticflickr.com/9/8565/1...6ed2ba81_b.jpg

What is almost incompressible to me is HOW the flame sensor
failed. It wasn't dirty. It wasn't shorted. It's just a steel
rod! The ceramic wasn't visibly cracked. I just don't get it,
but, it is what it is.

Congratulations on fixing the problem. However, from an education
point of view, your statement that the flame sensor is just a hunk of
stainless steel is NOT correct. Google "thermocouple" and see how
they work. The current does not go through the flame. It is
generated by two pieces of dissimilar metals creating a voltage which
then causes a tiny current to flow and be sensed. They can and do go
bad.