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so, I infer that if it went to ground, it would show infinte resistance, giving him a return of 0 volts.

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Incorrect. Infinite resistance would look like an open circuit (no sensor). Interesting side note: at -40F, the sensor has a resistance of ~100,000 ohms. This will give a voltage very close to the no sensor voltage. So, if you start up at -40, the fan may run for a minute until it warms up a little more

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however, the problem occurs when the ground goes away, ie open circuit.
This too would trip the fan on due to the returning 12 volts.
which is why there needs to be a switch to determin if you are reading from a sensor or a ground.

IF I understood correctly.

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Correct