2nd prius with p0138 code

Hi,everyone,I have a 2005 prius,recently it comes with checking light on,brought it to the shop they told me code is p0138 and let me buy downstream denso o2 sensor from autozone,I bought and they installed it,but after 2days,checking light come back again.brought to shop again,still code p0138,they told me maybe what I bought is not working,but I told the fuel efficiency is better,from 35mpg to 40mpg now,they still told me to buy new upstream and downstream sensor,I do not know what to do,does anyone encounter same situation before?what should I do ,thank you so much

 

23000miles

 

230000miles