I have a "Service Engine Soon" light on my instrument panel. My technician ran thru' the diagnostics and it said it has something to do with mixture being too lean in both banks of the cylinders (2000 540i). He checked around and found that the oil separator diaphragm was leaking air. So, I had that replaced as he was so sure that was the problem.
Now the same light comes on again! I suspect it could be the oxygen sensor. Has anybody had this problem before? If so, I hope I could have some tips or suggestions so that I could get straight to the problem instead of spending money replacing the wrong parts.