Check engine light came on , checked flash code is 21-oxygen sensor. I went in to pick one up and guys asks which one, upstream or downstream? He says I have to take my car somewhere to have them both tested. I would like to know how much I should expect to pay for the tests and installation. I know the part will be either $53 or $73. Also, does anyone know how much it can effect fuel efficiency and engine if I don't replace? Thanks in advance. K