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See the problem? One says no ground turns on the fan. The other says no ground, no fan. It would require logic to be added that determines what to do when either switch goes to ground or remains open and which to ignore.
A dipswitch would tell the controller which program to follow.
Exactly! Said much more elegantly than I could come up with 
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two, I was wondering if short to ground is a failure mode for temp sensors in gneral.
That is I understand open, IE no resistance at all, but what if it shorts to ground instead?
or is that a high resistance reading?
I'm not sure what a normal failure mode for the sensor itself would be. If it shorted to ground internally, the voltage I read would go to 0v, which would never happen normally, so I can turn the fan on. If it failed open (same as unplugging it), the voltage would go high, which would also never happen normally, so I again can turn the fan on.
The failure I was more concerned about was if/when the wiring got disturbed and the sensor came unhooked from the controller, assuming it's set up to look for the sensor. In that case I need to turn the fan on.
Using a switch to turn the fan on implies one of the above failure modes, so I need some way to know why a sensor isn't detected.
If you ever find yourself in a fair fight, your tactics suck.
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