Chances are the draft inducer motor is not pulling enough CFM through the heat exchanger.
That is usually due to either blocked/clogged flue pipe/vent.
damaged, missing or dirty fan blades.
or a motor going bad.
Check those three things first, as they are the usually problem. Other than that, it would be a sticking or bad switch.
If you s-uck on the end of the hose like a straw, while it is attached to the switch and it does not pull in then you are looking at a bad switch/cracked hose issue. If it does, then you are back to the draft inducer motor or it's squirrel cage fan being bad.
These are notorious for rusting and the blades breaking off.
1. Thermostat calls for heat.
2. Draft inducer motor starts.
3. Pressure switch attached by a small plastic or
rubber tube senses the negative pressure produced by
the draft inducer and closes.
4. Draft inducer runs for 30 seconds to a minute
before you hear a gas hissing sound. The ignitor did
not glow, the flame sensor (a small metal probe about
1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops
with no ignition of gas to heat your home. Your
furnace shuts down and goes into a lock out condition
until you turn your power switch back off and on
again. Then the sequence starts all over again with no ignition of the
gas.
Solution: You
probably need to purchase and install a new ignitor. I would suggest that you
inspect your ignitor closely for cracks. Make sure
you do not touch the ignitor with your bare hands. If
you do not visually see a crack, then you could have a furnace control board
problem or a limit, rollout switch problem. The furnace's
control board might not be supplying the voltage to the ignitor. If
your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right
back off, then you need to clean your flame sensor with light sand paper or
steel wool. You might need a new flame sensor, but most of the time they can be
cleaned an will work well after cleaning.
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