overhead problem

my friends car 206 1.6 roland garros lpgli 2001 model. when I start the car in the cold weather in the morning, the temperature hand is working at the end and the fan is activated. It goes back to normal after boarding for 15-20 minutes. I took it to the service, the teermostat changed. . They said the heat sensor. they changed it, but there was no solution. Do you have any friends? What do you think is the problem?