<advice needed> E46 emissions faulty

[Amir83]

Hi guys,i drive a 2001 E46 318 CI. Its a beauty! but lately i have this problem. There is this light that keeps appearing on the dashboard indicating that there is a problem with my emissions. The BWM booklet says that the light indicates "emissions faulty". I went to a BMW mechanic and after doing the diagnostics he suspected that it is the Oxygen sensor. I changed it, but after a day the light re-appeared! Anyone with similar experience? Please advice.Thanks