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Discussion Starter #1
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(Downstream on left, upstream on right has been cleaned up already)

On a long drive in my wife’s 2003 Altima 2.5, the car started bogging way down. I had just fixed a misfire that had been caused by a leaky valve cover allowing oil into the spark plug tube for cyl 2, but the car had been driven with the misfire for a WHILE before I could get down there to fix it, and I guess the whole time it was just dumping fuel into the catalytic converter. Even the previous owner who we got it from four months ago was driving it around like that.

Well, I pulled it over in some small podunk town, and pulled the upstream O2 sensor, thinking I’d let the motor have some outlet to exhale. The sensor on the right is the upstream, cleaned up. When I pulled it out it had carbon deposits just like the one on the left, just no soot on the bottom. I also noticed a small crack in the pre-cat just below the point where the exhaust manifold joins the cat, telling me that the exhaust leak I had been smelling prior to all this was probably right here.

Anyway, will gutting the precat and reinstalling it be my best bet? And how in the name of God did my upstream O2 sensor get blown out so bad? Thanks.
 

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You'll get a cat code.

O2 sensors don't blow out and based on the silver metal chipspot showing on the right hand sensor between the middle holes I'm guessing rough handling in yanking the part.
 

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Well if it was rough handling that broke it then it had to have been the previous owner getting it out and putting it back in place, cause I didn’t have a hard time with it, once I broke it loose it came out pretty easy.

I was actually wondering if the heating element inside of the sensor is known to blow out. I’ll take another picture when I leave work, because you can’t see it from the angle I took the first one, but you can clearly see a split down the side of the ceramic underneath where the metal shield blew out. I don’t know if the heating element extends that far into it or not.
 

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Wow, that's a first for me. Don't believe they're known to blow out like that. Could possibly be the heater core in the sensor I suppose...

I wouldn't even waste my time gutting the cat. Replace with a new converter and be done with it bro.
 

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No, still haven’t found out what was wrong with the sensor. I ended up gutting the cat over the weekend, broke one of the studs that clamp the outlet to the flex pipe, so I don’t have it all the way together just yet, and it’s loud af until I get enough time after work to drill it out.

I did put both the O2 pictures back in to see if it ran any different with a cleared cat, and with the upstream (exploded) one it ran like trash, just like it was when I originally thought it had a clogged cat, so now maybe I’m thinking it is just a bad O2 sensor, but how did it go bad like that?
 

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Well, I don’t think that the cat was fully plugged. Cause once I got everything back together it runs like trash unless I take the O2 sensor out. Which is the same thing it was doing before I cleared the pre cat. I even left the exhaust undone from the flex pipe all the way back after I cleared the pre cat, and it still acted like it would only run good with the O2 sensor pulled out of the cat, still hooked up but tied off to the battery stand.

And that leads me to believe that it’s something in the exhaust gas that’s making the O2 sensors (even the new one I just put in) adjust the fuel/air mixture in a way that’s bogging it down. Almost reminds me of a vacuum leak, actually.
 

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An intake leak would do that. Often it is the intake manifold gasket to the head that folks have troubles with.
 

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What upstream sensor are you using? If it is the exploded one, ore a cheap quality new one, that could be the problem. You need yo install a good quality upstream sensor. If you don't want to spend that amount of money and have any friend with Altima you can borrow his upstream sensor to see how your car reacts.
 

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I did order a cheap Walker replacement off Rockauto, that might’ve been my first mistake. Because now that it’s outside the exhaust flow it’s sending back a lean mixture code and it makes me think it’s dumping more fuel in there to even the air/fuel out, because it seems to be drinking gas a little bit more than before. But if the cat isn’t clogged it makes me wonder what caused it do to that now? Why all of the sudden would my O2 sensor being in position bog the motor down? But when I remove it and leave it wired in still with the hole sealed off it runs like it did before?

I just bought a little O2 WiFi attachment for it so I’ll keep checking the data and see what I can figure out.
 
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