I am a relative novice to auto maintenance but I need to have my O2 sensor's replaced and am going to do it myself (my mpg has been dipping over the past 1-2 years from around 26mpg city to currently 20mpg). I've located both the sensor on the manifold before the catalytic converter and the sensor after... do I need to replace both? As far as I know, neither have been replaced before and I'm creeping up on 90K miles. I'll also be cleaning my MAF sensor while I'm at it... any comments, tips or suggestions are welcomed.
While I'm at it - what other tune up suggestions above and beyond the normal "oil change" and "tire rotation" routine should I be looking to do? I'm enjoying learning about my car and doing some work to it. Ive went from being hands off to changing oil, rotating tires, changing rotors & brake pads.... and now the sensors. I'd like to learn more please.