I wouldn't replace the sensor at this stage.
Just see if its been bypassed.
BTW, it's probably mounted in on the rad end tanks.
If you want to check it: remove it (you'll lose your coolant though), dunk it in freshly boiled water to check to see if there is continuity across it with your multimeter.
If not, then definitely replace it.
If one of the sensors in your A/C is playing up, generally the A/C simply won't come on (ie the compressor won't engage)