The Thermostat sensor isn't a sensor if you're referring to the one on top of the thermostat housing. It's a temperature controlled switch that turns on power to a relay that turns on the cooling fan. The real temperature sensor is on the back of the head.
Have you replaced the thermostat yet? That's the biggest culprit.
Don't go cheap, get the safety model thermostat. It's only a couple of bucks more.