I had a bad o2 sensor (the one after the CAT). The code was showing this and the voltage signal confirmed it. I drove it for about a month with it in this condition. After replacing the sensor the engine light came back on after aprox 10km. Checked codes again and it is saying that there is a fuel trim problem and 02 sensor before the CAT. The codes were reset and the engine was monitored while driving, and a the mass air had a little hick-up at highway speed causing a code to store in bank 2 but not causing the check engine light to appear. The engine light was coming on when the o2 and catilitic efficiency codes appeared.

I don't want to keep replacing $300 o2 sensors or $1000+ parts (cat or mass air sensor) if the root problem can't be found.

Any help please and thank you.