The blades on my Troy Bilt 46 inch garden tractor will not disengage when I push the PTO knob in, how can I troubleshoot what is wrong with it?

Hi,
My guess is that one side of the switch is shorted. It has two circuits in it. An ineffectual check would be to rise up off the seat to see if the machine tries to shut down. If it does not, you can bet it is the switch. If it does, you can guess it is still the switch.
Peace,
Clarke