On Sat, 10 Jan 2015 00:11:04 +0000 (UTC), "Danny D."
wrote:

5. However, replacing the flame sensor worked.

How the flame sensor failed is a mystery, as it clearly was not
tarnished (it was sanded with steel wool, emery cloth, and a file,
over subsequent attempts). The ceramic "looked" intact, but may
have developed a fault such as a crack.

The assumption is that the flame sensor failed, although the
only proof of that is that the new flame sensor worked as
soon as it was installed.

Thanks for all your help and advice.

Doesn't the flame sensor have a "special" metallic coating? Sanding
it off will damage the sensor?

Who knew that the flames form part of an electrical circuit!

I think my HVAC furnace has a safety feature. When the flame goes out
the gas shuts down the gas valve. Wonder if it via the sensor?