The sensor is a resistor that changes resistance with temperature. I'd put it in a voltage divider so I can read the voltage, and from that calculate the resistance and the temperature. Without the sensor hooked up, the voltage will read higher than it would with the sensor, so I can detect that. I can also detect if it's shorted to ground.

The problem is I have no way of knowing, if the sensor input is high, if it's because the sensor isn't hooked up, or if it's waiting to be grounded. Adding another 'grounding' input would require a jumper or resistor put in place of the sensor.

So I think the only solution would be a dipswitch to change modes. Unless we get rid of the no sensor fail safe (which I don't really want to do).