Replacing oxygen sensor
 

I have a 97 Plymouth Breeze 2.4L and a few weeks ago the 'Check Engine' light came on, stayed on for about a week, and went off. While the light was on I managed to get the mill codes which told me there was an o2 sensor issue (now I know mill codes can't be reliable and I should have the p codes) Ever since then I have noticed a considerable difference in my fuel economy, and there is a very rough idle when in drive (steering wheel vibrates considerably) I was wondering If I should replace both the upstream and downstream O2 sensors or just one, if so which one??

Thanks in advance,

Trippo

replace them both and not with a bosh o2 sensor either