The thermostat isn't on the back of the 3400.... it's just under the TB which is on the driver's side of the motor. Best way to check if it's faulty is to remove it and throw it into boiling water and see if it opens up. It's not a sensor, it's strictly a mechanical device. I had mine go on me in the middle of the winter and it was stuck open so my engine coolant wouldn't get warm and I wouldn't get heat.
This video helped me:
http://www.youtube.com/watch?v=ZgLWR1dCXRM