I replaced my front bank O2 sensor after reading code 21. Just now while driving I get a CEL. I seem to be getting these CELs only after warming up. Bad CTS?

I see from another post that it would probably be the rear O2 sensor. I knew I should have bought 2. Is that sensor accessible through the wheel well?

"I looked up the codes, and alldata indicates that it is a O2 sensor fault. For 25 it could be the "main sensor low for 90 sec" or the the "difference between right and left bank more than 15%".
26 Also talks about the difference between the two banks. So I would recomend swapping both sensors."

 

maybe a cts or coolant low / massive bubles in it but most likely just a oxygen sensor or two needs to be replaced

its accessible [the rear one] only from underneath [not quiet from wheel wheel like on say a mazda v6 ].
uses two nuts to hold it on.


could be other stuff but you have to rules stuff out. 02 sensor are 80K maintenance item that most ppl just wait to change when they are so bad the ecu codes ti