Check Light Error

I am Facing this problem please help me! check light comes mechanic told me your oxygen sensor bank 1 is faulty replace it. I am from pakistan it is not avaible in new but used I got It I installed after running 100 km check light again come it is so that if i got new one what is gurantee light will not come my concern is if Oxygen Sensor is faulty then why check light comes after 100km why not it instantly check light come when I reset check light through OBD Scanner I search the error code it shows oxygen sensor 1 Faulty.