Limit switch for heat exchanger open should be closed ,heat up with a lighter til you hear a click then test.
Pressure switch. see if filter is clean
SOURCE: armstrong ultra sx 80 furnace
Flame sensor could be dirty. It is located right beside the last burner looks like a thick piece of wire. Take steel wool and gently clean the probe 4 or 5 strokes is all it will take to take off the carbon build up off. Lets eliminate this item and see if it clears up okay. ken
SOURCE: Rheem gas furnace pilot lights, but no burners or fan
If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.
SOURCE: armstrong ultra sx 80 furnace
You have a dirty flame sensor. Look at the very last burner and you will see a wire going to a ceramic sensor and it looks like a thick wire. Take some steel wool or fine sandpaper and clean the surface off and it will work like a champ.
Let me know how it goes ken
SOURCE: How do I clean a flame sensor on a 1993 Lennox
You can sand the flame sensor but it sounds like another problem. Sounds like a limit switch or something in the control circuit, You need to have a service tech look at this. Ask for free estimate on repair. I have worked on hundreds of these units and they are sometimes tricky to work on. Lenox has come up with new controls for the whisper heat furnace to replace the old ones. If the flame sensor is bad the unit will try to light a few times and lock out and you ahve to kull power to the furnace to reset. If this is whats happening clean the sensor nd you should be ok If not you need help from a service tech.Rus
SOURCE: FURNACE COMES ON AND SHUTS DOWN WITHIN 15 SECONDS
On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.
What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control). When all else fails, check the ground connection for corrosion.
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