I have a Rheem 90 gas furnace. It didn't light up over the
weekend, so called service. Well you know it, right before
he got there, it worked. He couldn't tell what the problem
was at the time, and it stopped firing again shortly after.

I check for 24 volts at the gas sensor, and there was none,
and according to him, this would indicate a bad circuit board.

But I check the pressure switch that is between the gas valve
and circuit board, blew some air through the vacuum hose, and
now it's working.

Is the problem with the vent(or drain, don't recall) pressure switch,
not sending a signal to the gas valve? Or was there some obstruction
in the pressure lines?

Any help would be appreciated!

 

They get obstructed occasionally. Sometimes you will have a condensate problem and water will build up in it and that is normally fixed by unclogging the condesate drain.