The interior temperature sensor is in the dome lamp housing. It's a small grille next to the sunroof switch (if you have one).
I'm not sure if this is the problem, but here is a simple test you can do.
The first thing you want to check is the ventilation blower for the temperature sensor. Get a small piece of paper 1 cm2 and place it over the grille. Make sure that the ignition is on. The paper should stay in place. If not, remove the glove box and check the small blower it's attached to the body on the right side. If it works you need to check the tubing for leeks.
To test the actual sensor is a bit more complicated and should be performed by a tech.