1999 Pontiac Bonneville Q&A

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

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!! -
Answer 1
Have you scanned it and verified the code is the o2. -