My 2010 Mazda3 with 100k miles runs like a clock. But last week the check engine light came on with the same PO138 (high circuit o2 sensor bank 1 sensor 2) code. It disappeared the next day and has not come back since. But this is the second time this year the light came on for the same code (happened during spring and it went away until now). I checked the short term fuel trim and it’s between 1.0 and 0 and -1.0. Also the long term fuel trim was at 6.3. I thought the fuel injectors were probably clogged so I used royal purple max clean fuel injector cleaner to clean them. The car felt a little more responsive than before, but I checked the long term fuel trim next day and it went up to 7.0. After a short drive that day it went down to 3.1. I checked again two days later after a day of driving for work and the long term fuel trim was at 8.6. Short term fuel trim is still the same the first time I checked it. What is going on? The car runs great but I use the it for work and I’m trying to prevent any expensive repairs in the future. Also the exhaust has a noticeable smell coming from it and I think i need to replace the air filter since it’s a year old and it’s kinda dirty.

it’s a year old.