My armstrong ultra sx 80 furnace will not ignite. I have replaced the dirty filter, but the fan stay on all the time, without igniting the pilot. I have already checked out the rubber tube connected to the pressure switch/ combustion area. There is no rust or clog, and when I blow in the tube I can hear the valve diaphragm moving. Is there a sensor that might need to be cleaned? The unit is only 7 yrs old and has never before been a problem.

I need more information.

Look on the back side of the furnace blower motor acces door for a wiring diagram. Look on the diagram for the high limit safety then follow the wires on the diagram to the control. There should be a very small reset button on the control. Push to reset and try to operate the furnace.

NOTE make sure to turn off the power before you reset control for your safety.