Hi, first time on this forum...I have read many things so far and I know after reading many posts that I will get an answer...There are many great minds here...

I have a 1999 Bonneville SE.......I took it to Autozone and they told me I had bad 02 sensor....I bought a sensor not knowing there was 2 sensors on the car...I replaced the one that was before the cat converter....

Check engine light still came on and the car was running just as bad....

Here is the question....Do I put the old O2 sensor back on and take the other back and get the sensor after the converter?....

Does a faulty sensor after the converter make the car run bad or is it telling me that the cat is bad....I tried shaking the cat a tad but it was solid and made no noises.....Can someone give me advice?.....Thanks so much!!!

You should have 2 O2 Sensors before the cat. I would go ahead and change the other one also. The one after the cat is just used to monitor your exhaust to ensure the cat is working and for performing an emission test. A bad one after the cat would not affect the car performance.

There are 3 sensors?.....I changed the 1 right by the rear spark plugs that sits upright....I also changed the one that sits on the tail of the cat converter...Where is the third one?....The scanner says that it was the downstream sensor.....Where is this third one located...Thanks azu for your help....

Downstream O2 is after the cat and that monitors the cat'* efficency and exhaust emissions. You may only have one "upstream" o2. To tell you would have one in the front exhaust manifold as well as the rear one you already changed. Still the downstream sensor should have no bearing on engine operation. It mainly tells you the cat is possibly bad and emissions are high. That may be your problem. Your cat could be partialy clogged. I myself have to get one soon.


Also after you replaced these did you have the codes erased? If not you can disconnect battery for 15 minutes to clear the codes.

What make sensors did you get?

THANKS so much for getting back to me....It was the sensor that was on the cat that was bad...I disconnected the postive and the code hasnt come back for almost 300 miles....

The car still runs like crap...I changed all the plugs....I do believe it is the cat thats failing....Is there way to check it?.....or was the sensor just telling me what was the truth...The sensor had lots of black stuff on it.....

I found a direct fit one on the web for 140.00 bucks...I think Im going to go ahead and change it this weekend.....

Any more suggestions Azu?...Thanks for your time......

I did get the Bosch sensor for the downstream sensor and Delco for the upstream....

In the future, have them scan it and give you the code. I wouldn't buy parts until I googled or asked a forum about their experience with codes.

Autozone wanted to sell me a sensor for a code that was being thrown because the sensor was doing it'* job. They are guessing at best on what the code means.

No Bonne to my knowledge has three O2'*. All have one in the rear manifold and one after the cat.