Engine code problems. Please help...
 

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.

What codes are being triggered? It may be related to the MAF or vacum hoses. I recently had the P0170 and P0173 codes. I am going to try and clean my MAF, if that doesn't work then may need to replace it. Search the board with the code and you may get some answers.

I asked my mechanic and he said the codes are 234 cat efficiency bank 2 deminished. This code is coming up on a regular basis. Code 226 fuel trim problem has also been logged. He uses a Snap-On scanner which gives the BMW codes, not a genearic code.

If you have no Vac leaks the MAF is probably the starting point. The o2 code is probably a result of the fuel trim.