Gas or electric? If gas make sure the line is on by checking the valve. If electric check your breaker panel box and see if it is tripped. If so reset the breaker.
2 flashes = pressure switch stuck open.
It sounds like your
inducer fan (furnace gas exhaust) is running. The circuit board will verify that
this fan is running using a pressure switch that is connected by a silicone hose to a
port in the fan/blower housing. This pressure switch (they call it a
pressure switch but it really is measuring vacuum) is a 3" round, flat dash-pot switch that has two wires
(typically orange) connected to it. Here are the things that can go
wrong:
Common: Make sure the silicone hose is intact and not leaking. If the
end connected to the blower housing is rotten/broken/crumbling from the
heat then trim it off and reconnect.
Common: Remove the silicone hose from the blower housing and carefully
stick a drill bit into the port trying to scrape away the crusty residue
that builds up all over the exhaust of these furnaces. Crusty stuff on
the inside edges of this port can alter the venturi effect making the
vacuum signal lower and ineffective.
Common if: If your blower housing was built with a coating of paint
inside it, that paint can come loose and put a drag on the blower wheel
just enough to lower the vacuum signal at the venturi port so that the
pressure (vacuum) switch won\'t close.
Note: if the crusty stuff or loose paint condition is causing this you
may find that the furnace starts and runs for a short time (a minute)
then shuts down. That is because the cold dense air is creating a barely
good enough vacuum signal but once the furnace is running the air has
gotten hot in the blower, which makes it less dense, which is not enough
to keep the switch closed so the furnace shuts down.
Turn off everything, disconnect the two wires at the pressure (vacuum)
switch and connect an Ohm meter and very, very gently suck on the vacuum
hose. This is a sensitive switch that only takes 2" of water column to
activate ( the amount of suction it takes to draw water 2" up a straw! )
so be gentle! The Ohm meter should go from infinite (no continuity) to
about zero (contact or continuity).
If all the above works you may have to verify the suction available at
the blower port and the actual vacuum signal needed to turn on the
pressure (vacuum) switch using a manometer. You can make a water
manometer with a ruler and a long length of clear vinyl tubing the same
size as the silicone tubing, and some water.
If all this checks out then replace the controller board.
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