"Service Engine Soon"

[CosmosBlack]

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.

Thanks!

[jhkoenig]

Simple/stupid answer: the gas cap may be loose. The captive leash attaching the cap to the car gets trapped under the cap as it screws on (at least on my 2000) which triggers the SES light.

Once I freed the leash and tightened the cap the light went out. wow ghead

[CosmosBlack]

Thanks, that is one of the causes but usually not so, unfortunately. I wish it was then it would be cheap! I have fixed the problem since this post. Replaced the throttle body and Mass Airflow sensor. Both jobs totaled almost $900.