Whenever the system sees O ohms in the temperature sensor loop it causes the servo to go to full heat (the good news is at least you know your servo is working). Try putting the sensor in the freezer -when you take it out wrap it with a cold pack and try plugging it back into the system to see if that triggers it to serve up some heat. This test will show if the thermistor is working, but just out of range. This is not a definitive diagnosis of your problem as your amplifier may be out of whack.
If you do need a new sensor, RadioShack will not have a thermistor that matches the temperature/resistance curve. The reason I know this is because several years ago I attempted to do the same thing. I obtained the chart that shows the resistance vs. temperature curve on the sensor -- unfortunately I was not able to substitute it from another source.