I have a furnace that keeps locking out for an hour at a time when it
returns to normal until the next lock-out occcurs. My furnace
contractor has replaced the control board twice, the gas-valve once,
and put in three new flame sensors. They now are wondering if my LP gas
could be at the root of the problem because the flame detectors have a
green tinge to them.

I have two questions:

1) The ignition failures that lead to the lock-outs do not involve any
flame. That is, they occur before ignition; the gas just doesn't seem
to flow and the ignitor is definitely on. If the flame sensor was the
problem, wouldn't the failure occur after the flame was present, that
is it would not detect the flame even though one was actually present?
Isn't the flame detector's only function to detect for absence of flame
when gas is flowing into the burners and to shut off the gas at that
point?

2) Assuming it is the LP gas, what could be wrong with it? What sort
of adulteration or impurities should be looked for? If it's a problem
with the gas, why would it work most of the time and not others? The
first failure followed the last fill date by three months. The second
failure followed the next fill date by three weeks. Now, on the same
tankful, lockouts are increasing in frequency to four or more a day.

I'd greatly appreciate any suggestions on what could be wrong. I'm at a
loss.

Thanks.