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Thank you...is the exhaust fan running?
The code is saying that the air created by the exhaust fan is not closing a pressure/air switch.
Most common issue is blockage in vent pipe.
Next is small rubber tube is cracked or plugged
Then the fitting on the furnace the tube attaches needs to be poked clear.
Basically we are not getting suction on the tube (like a straw) to close this air switch. Do you know where the pressure switch is?
depending on if your furnace blows up, down or sideways...The pressure switch is near the exhaust fan and has a rubber tube as thick as a pencil to it.
are your pipes to the outside clear? you can use a shop vac if needed
if drain was filled we may have water logged switches...remove the tube from that goes to pressure switches from the furnace side...leave attached to switch. Then gently blow and draw from the pressure switch through the tube (like a straw)..tell me if you here small clicks from switch as you do this.
Tell me what happens when:
Turn thermostat to off or turn temp down to where there is no call for heat.
Turn power off and back on to furnace.
Does the light stay solid?
Turn heat on....when does the light start blinking?
Does the exhaust fan blow good amount of air out when running?