overheard reading read?

Hello friends, I have a 2006 model 206 1.4 (75 hp). I have a problem with overheating for about 2 months, the temperature does not exceed 70 without bringing my car to high revs or going into heavy stop-and-go traffic. I have a problem with fuel and I think the reason is this heat problem, because the normal maintenance + heavy maintenance (timing, circulation) of the vehicle has just been done.

It looks like a thermostat failure. It may not have been left open or plugged in at all. When the temperature is around 70, open the hood and touch the radiator. If the radiator is heated, the exact thermostat is defective.

the thermostat and temperature sensor have just changed, but the thermostat billet has not changed. I do not know exactly what the task of the billet is. Is there a possibility that it will fail. Is there a section like valve or valve that can stay open. I guess it is necessary to investigate, there is no other mechanic that controls the circulation of the water.