Smart gasoline is not getting warm..

Hello together, I have with my Smart (small engine) the problem that it just does not get warm. Once the indicator for the cooling water temperature goes up after 30 km overland but also only to the second point, and after motorway rides it goes back from the third point to the second point. Not to mention the cold that comes from the ventilation… What can that be? Maybe the thermostat is defective? Has anyone already experienced this problem?