Try testing the thermostat with the meter (on resistance ohms)  on the centre tag plus the outer ones in turn.
I think that you should get a short circuit on one tag , and open circuit the other way. 
Then heat the themostat up (with a hair dryer or drop into scalding water) then retest, and the opposite should happen as the thermostat switches at the hotter temperature.
If it doesn`t then change it.
                
 daz
                
 June 2008