changing o2 sensor

[I_love_my_TSI]

Hey fellas this is my first time Im gonna change my O2 Sensor...But I notice there is Two O2 sensor... There is one in the Front and one in the Back toward the end of the exhuast..Which one is more important to change..

 

[I-luv-em-all]

A little more info would be nice. But why are you gonna change the o2 sensor anyway? Just for the heck of it? or are you getting a "CEL"? Bad gas milage? Puffin some black smoke?

 

[I_love_my_TSI]

Im just getting poor gas miles, and the car is about 90,000 miles on it ,,