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1. Obstructions or restrictions
in appliance air intake or flue
outlet are preventing proper
2. Moisture or debris in tubing that
connects pressure switch and
3. Airflow switch jumpered or
f this is a metal vent 80% furnace,Turn Furnace off and remove the vent that connects to furnace and see if it is plugged.
If this is a High Eff furnace witrh pvc vents for exhaust and intake air make sure they are not plugged on the outside.
Turn thermostat all the way up,turn furnace off and wait a few minutes.This should reset any control board fualt.
Turn the furnace back on, if you have a glow coil ignitor it should glow red and then the burners should ignite. If this doesnt happen then:
On High Eff Furnace if the glow coil does not glow red, remove the combustion door to the burners, turn furnace back on, if the glow coil glows and burner ignites, you have an obstruction in the intake air side. If it still does nothing then:
Use caution and remove the exhaust vent from furnace, start furnace, if furnace runs then you have a problem with the vent system
i take it the flash code 3 blinks refers you to the pressure switch...see the schematic. If this is what you are saying than lets move on... the voltage across the pressure switch should read 0 if the furnace is running and calling for heat. you dont need to check each wire to ground but across the two quick connect tabs. you will either read 0 or 24 volts (or there about). if you read 0 the switch is closed and functioning as designed. if it reads 24 volts it is open and there is not enough negitive pressure to close it. this can be due to a leak in the rubber tubing or a clog in the pressure switch, tubing, or induced fan housing connection nipple. the flue pipe may also be clogged or an improper cap is used at the termination. last is the wiring could be faulty going to and from the pressure switch. Hardly ever have we found a broken or faulty switch but it could happen. trust the board in most cases.
Your induce fan motor senor is out. The little fan motor that take out the heat has a sensor that tells the furnace thats its working. Some time it get stuck. Try blowing and sucking into the tube. you should hear a click
the switch should be mounted in the same area as the burners. there should be a couple of hoses that run to it. most likely is that the intake or exhaust is plugged with frost or there could be standing water inside one of those pipes that has started to sag due to improper bracing and support. in other words, check the venting of the furnace. occasionally water can collect in the small tubes going to the pressure switch. the furnace drain has a tendency to plug up with gunk especially in the trap( a little black or gray plastic box with several hoses attached to it). it will most likely be in one of those areas.
ok this is going to sound funny but it seems as though the polarity is reversed going into your furnance. to correct this proiblem you first need to unplug the unit and where the cord goes into the furnance at that connection disconnect the two wires. Once theses are disconnected switch the wires .All you are doing is reversing the way the electricity flows into the circut board. I would wager that is the problem
check the pressue switch and make sure that nothing is in the fan that would cause it to not draw. I have seen water inside of them at times due to the high efficentcy of the units. let me know if this does not solve it please.