SMS wrote:
The electric ignitor always starts up correctly (it's not a spark
igniter, it's more like a glow plug).

Sometimes the solenoid for the the gas valve opens and the burners
ignite the main blower comes on and it works fine for about five
minutes.
Even when this all works properly, after about five minutes of heating
the burners go out, then the controller tries four times to light
them.
When the burners go out, the 24V on the gas valve connections goes
away so the gas valve closes. But when the controller tries to
restart the burners, the ingiter does come on, then there is 24V on
the gas valve, but the gas valve doesn't open (at least I don't smell
gas) and the burners don't ignite.

What is it that is tripping and thinking that the gas needs to be shut
down in the first place? Why doesn't the gas turn back on when the 24V
is on the gas valve terminals?

I don't think it's the flame sensor, because the gas should come on
when the burners try to be ignited then the controller only turns off
the gas if the flame sensor doesn't detect a flame.

The draft inducer motor runs fine, and pulling off the hose, there is
air coming out. I bypassed the pressure sensing switch (you have to
have in open in order for the system to start, then closed once the
draft inducer is running) to see if it was failing, and even with it
bypassed the gas still shuts off long before the thermostat
temperature is reached.
I bypassed the three firebox flame-out sensors (one by one) and it
didn't help (I did not try bypassing all three at a time).

Any ideas? I would say it's the gas valve but since it works fine, one
time, I wonder how likely intermittent operation is.

The flashing trouble code LED just indicates "fail to ignite," and
indicates that the trouble could be the gas valve, the gas pressure
from the supply, or the flame sensor.

It's an old Sears Furnace, probably 25 years old. I can't find the
model number anywhere. The gas valve is a White-Rodgers 36E36 227.
How hard is it to replace these. It looks like just removing the
pipes from the gas supply and the manifold. I'm just worried about
damaging the manifold if it's really tight.

I wonder if it is the electric ignitor that is the problem, even though the
ignitor glows and appears to be working.

The reason that I wonder about that is that I had a similar problem with a
gas oven that wouldn't heat up. The ignitor appeared to be working because
it glowed when the oven was turned on. I did some Google searching and
learned that even though the ignitor was glowing, it still could be the
cause of the problem. At least with a gas oven, the ignitor has to allow a
certain amount of current to go through before the gas valve will open up so
the oven can be ignited. If a gas furnace has a similar setup (I don't know
if it does), then maybe the ignitor is the problem -- despite any voltage
readings that you may be getting. Apparently, it is not just voltage (in
the case of a gas oven), but it also requires a certain amount of current
(amperage) to be drawn before the gas valve will open or open fully.

Here are two links about that in regard to gas ovens:
http://www.applianceblog.com/mainfor...ont-light.html



http://www.youtube.com/watch?v=zkV4Wj0klkQ



Again, I don't know if a gas oven and a gas furnace have a similar setup,
but I thought I'd pass this on in case it helps.



I also just tried a little Google search regarding furnace ignitors and this
link has a post by "Kokomo" that says the only way to test the furnace
ignitor is by measuring the current (amperage) draw:



http://www.doityourself.com/forum/ga...e-igniter.html .



Good luck and let us know what the problem turns out to be.