The fume switch automatically opens when you lose the draft. When it does, it should shut down all the motors: stoker, combustion and vent. The furnace then goes cold. Once the draft problem is solved, the switch can be reset by depressing the red button in the center and the furnace restarted.
The problem we've been hearing about is the Koker's wiring is such that the vent motor has been left to run while shutting the other motors down. This draws air thru the coal in the hopper. The fire follows the air path up into the hopper, melting the nylon cam along the way.