I have an older coleman /york furnace and after the heat has reached the tstat setting the burner goes off but the fan never stops. I have to manually turn off the power to the unit. The model # is P3VGD12N08001B

Is the fan setting on the thermostat on "auto", or "on". Should be on auto. If its on "on" the fan will run continuously. If that isn't the problem, you either have a bad fan control in the furnace(many different types) or a bad thermostat or t stat wiring.