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Remove the rubber hose from the combustion fan motor.
Stick a long thin nail or a paper clip in the connection at the combustion fan motor. Make sure the nail goes all the way through touching the fan blades.
Apply a little force if necessary to make sure this connection is not blocked.
Also go outside and make sure the flue vent pipes are not blocked with debris, birds nest, wasp nest, etc...
If this does not correct the problem, then you will need a new pressure switch. I believe you call it the air switch.
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The flue is not blocked. Do you want the nail in the brass connector at the top or the black plastic coupler at the bottom??
It looks like the paperclip trick worked. Thanks