so let me get this straight.

you are applying voltage to the sensor line?
your expectation is if the sensor fails it will not read any resistance?
What if it fails and goes to ground? becomes an open short?
I don't know the typical failure of the sensors you chose so just asking.

so your logic only allows one choice?

so if line goes to 12 volts turn on fan.
Or switch, if line goes to ground turn on fan?

What I was asking is if you could have a seperate input.
so if line 1 goes to 12 volts turn on fan. or if line 1 reads x volts turn on fan.

if line 2 goes to ground turn on fan.


What does the sensor output look like to you?