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Sequence of events
1) thermostat calls for heat and sends signal to furnace to start draft inducer.
2) Draft inducer starts and pressure switch closes turning on the hot surface igniter.
3) Once igniter is glowing, gas vale opens and main burners kick on.
4) When the heat exchanger reaches a certain temperature, clixon switch closes and starts blower motor.
5) When thermostat is satisfied, burners turn off but blower motor continues to run until temp in heat exchanger compartment is low.
As others have stated, could be a bad relay on the control board or a bad clixon switch
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