On Wednesday, January 4, 2017 at 7:38:48 AM UTC-5, Ignoramus10081 wrote:
I have a Tempstar natural gas furnace. It has electric element for
ignition of the pilot.

It was working fine all December. Now, it does not work as follows:
the draft fan starts and runs correctly, then a good looking pilot
light comes up. In less than one second, the pilot light turns off and
the main burner does not ignite.

I don't know how your particular furnace works. Mine, there is
no pilot light, there is an igniter which just lights the main
burners. But given the behavior, if the flame detector is the
rod type, which is sounds like it is, that would be my first
suspect. They are basically just a metal rod, but I've heard
plenty of stories of strange behavior, where they would not
properly detect a flame and replacing it solved it. They are
also usually not that expensive and easy to replace.

Usually these furnaces have some LEDs that give an error code
that can be probative too.





I spent a while troubleshooting this issue. I cleaned the pilot sensor
with light emery cloth. It did not seem to help. It seems that if I
remove two screws holding the pilot light in place and pull it out of
the furnace, it does ignite and hold, and if I insert it back into the
furnace, the main gas ignites.

I put it back in and everything seems to work. I turn the furnace off,
then on, and the pilot stays and the burner lights. Then I went to
sleep and again woke up in a cold house.

Overall this behavior seems to be somehow "erratic". As in, things
seem to work, then after a while the pilot starts going away again.

Would there be some coherent explanation of this behavior.