OK, good news and bad. Check the actual fan speed potentiometer with a ohmmeter. There are only two outs from the pot itself to the little circuit board that its attached too. The pot is 75K so make sure your ohmmeter is in the right range setting. With the meter connected, turn the dial and make sure it goes from 0 to between 70K and 75K ohms. If it doesnt or its all over the board, then the pot is the problem. Now I can't seem to find one that is a direct replacement, 75K is an oddball. I didn't un-solder mine from the board. If yours is bad, unsolder it and look on the back for the manufacturers part number, google that with the p/n as the search terms.
The circuit for the fan is not tied to the sanatize button. The circuit is relatively small, with two film capacitors, two resistors, a diode, and a triac. The circuit is on the board located underneath the unit. The circuit is located right next to the fan pot inputs on the main board. Its about 1" square with the "fan pot" inputs at the bottom left corner, if reading labels right-side-up. Resistors are almost indestructable so it is probably not them. I would lean towards the triac, diode, or caps. Look at the components and the solder connects in that circuit. If anything looks burned or black, or a solder joint looks like it has gotten too hot, that component is probably the problem. If its not the pot, which more likely than not it probably is, then I would lean towards the triac or diode. Here is the triac p/n:511-BTB06-600T
. The diode I would also have to unsolder to access the part number, or I would list that as well.
I would start at the pot. That would be my guess as to what went bad. It could also be a bad solder joint on the small board the pot is soldered on to. Also check and clean the 1/4" disconnect terminals on the fan motor itself. For mine, I just cut off the old female terminals on the wires and crimped on new ones.
Hope this can give some help.