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this sounds more like the oxygen sensor is tripping. The vent pipe to out side...it is a pipe inside a pipe....the center pipe exhausts gasses...the outer pipe area between the 2 pipes draws air in for the flame...if the hollow area around the center pipe gets plugged, nest, sealed etc....it blocks air for the burner....and the result is shut down. Can you remove the vent cover outside and check that hollow area around center pipe?