Distance from ultrasonic sensor

#1

my ultrasonic sensor returns wrong values. I start close to it and the value is correct. I go progressively far from it. After a few correct measurment, whne I go 80-100 cm far it returns 400 .
ANy idea?

#2

Thanks correct. 80-100cm is as far as the sensor can measure. 400 is the value for nothing detected.