Would a new O2 sensor improve mpg on 2003 mini cooper s?

Answer 1

Yes & no in certain circumstances.

As o2 sensors age, the signal they produce becomes lazy (doesn't fluctuate as much). A lazy signal is inaccurate & the PCM will adjust based on the lazy signal. The PCM doesn't know any better, it just adjust the air/fuel based on the signal from the O2 sensor.

Now o2 sensors can go bad or just get worn out. A bad sensor will likely turn the check engine light on. A worn out one may be hindering gas mileage but not sufficient enough for the check engine light to come on.

If you have over 120k miles with the original o2 sensor, then I'd suggest to change it. You may get an increase in mpg