WHAT ARE SOME CAUSES FOR THE CHECK ENGINE LIGHT ON?AND WHICH SENORS GO BAD FIRST on 1999 Pontiac Bonneville

I HAVE REPLACED UP-STREAM OXYGEN SENSOR ALREADY AND NOW WHEN I HAD IT JUSTED IT SAID DOWN-STREAM , EVAC SENSOR I HAVE NO IDEAD WHAT TO DO!!

1 answer
Have you scanned it and verified the code is the o2.