Cleaning could be efficient for old not heated sensors where soot was coating platinum catalyst. New heated sensors they are heating ceramic sensing element above 300 o C and soot should be burnt totally. Gas contains always certain amount of mineral matter which is producing ash when burned. The main component of ash is silica which is coating the platinum element of sensor decreasing its efficiency. There is rather no way to clean this ash without damaging the sensor. I would replace each sensors after 100,000 -120,00 miles. I did experiment and found that sensor after 110,000 was responding much slower to change of the amount of oxygen than new one. The OBDII system did not complain but definitely the sensor was not performing as new one.