Not sure what a heat gun is. Will a 1500 watt hair dryer do the trick?
It might not get hot enough, as it should be designed not to burn your hair
Search for "heat gun" on amazon.com, I'd go for something like http://www.amazon.com/Porter-Cable-...UTF8&qid=1342368553&sr=8-17&keywords=heat+gun
which has a variable temperature setting. The heat switch might activate anywhere between 60°C and 120°C, you'd have to check the data sheet if there's no temperature indicated on the device (and if it's a switch at all).
For testing, and as it currently has no continuity, you could simply short the sensor and see if it makes any difference.
With best regards