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Discussion Starter · #1 ·
Just bought a 2008 2.5S a week ago. Car has 177,xxx miles and is still on 1st tank of gas. SES was on when I looked and he said it was a code for a loose gas cap. Looked great and drove great so I bought it.

It was showing he was averaging 21.6mpg and I was hoping for better. Tried checking codes with my scanner but it wouldn't work on it. Went to Autozone and they scanned it. Came back as upstream O2 sensor. Nobody had 1 locally so I pulled it, cleaned it, and reinstalled. Code immediately went off and was showing 25-27mpg hwy. Went ahead and ordered O2 sensor online. About 3 days later, SES came back on. Went ahead and bought newer code scanner and again, it was showing upstream O2 sensor.

New sensor came in yesterday so I changed it out and reset computer. MPG went up to 32 mpg in same area and thought problem was resolved. Wife drove car today and came in saying light came back on. Scanned code and it is now showing P0420, which I know is the pre-cat. I am also aware of the problems with this engine starting in 02 with the pre-cats. Went ahead and cleared code and figured I would give it a few days to see what happens.

My question now is whether I need to worry about the pre-cat issues of the past with this engine. I am not in a state that does inspections, but don't want to worry with having a light that stays on and not knowing of any new issues. MPG is now running over 34 on the hwy and car is running great.

Any suggestions? TIA
 

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09 Altima 3.5SE 6MT Sedan
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6,365 Posts
Actually it may not be the precat. The same thing that 'damaged' the upstream sensor could have damaged the downstream one...ie using leaded gas. You may find that replacing the lower sensor (downstream) fixes the problem. The code is thrown when up and downstream sensors send reading that don't make sense to the ECU...so if you have aftermarket upstream sensor and OEM downstream...that might cause an issue...if the old up sensor was contaminated...the lower might be too. You might just want to have your precat pulled and inspected and go from there.

Keep an eye on your oil consumption...if the engine is gone, you'll know very quickly by the dropping oil level.
 

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Discussion Starter · #3 ·
Thanks for the reply. The upstream O2 sensor I replaced was probably the original. I replaced it with a Denso which from what I understand, is the OE brand. As for leaded gas, I haven't seen that since the mid 80's. Don't think that has been used in it.

I have been my own mechanic for the last 30+ years. I do all my own repairs from tune ups to complete rebuilds. I just don't trust $tealerships or any other shops.

Considering the fuel economy has increased by 10mpg since the sensor was changed, I would think it was truly bad. Hopefully, any residue downstream may burn up since engine is running leaner now. I am just concerned about the pre-cat breaking down and getting back into the cylinders. I know it was what killed out most of the earlier models but was hoping that issue has been resolved by now.
 

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09 Altima 3.5SE 6MT Sedan
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Don't disagree. Maybe best thing to do is drive it for a tank of gas, including some long highway runs, and see if the code clears (probably have a lot of carbon in there). It has mentioned a few times that the probability of precat failure was worst 02-04, better 05-06, and 07+ definitely lower, at least as far as Altimas go. Hopefully it was just the O2 sensor--and kudos for going OEM on that...these engines are picky about their parts.
 

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Worst case, you do a header mod to eliminate the bad pre-catted manifold. Always nice to have some extra ponies and no worries of it to ruin your engine, while breathing better!
 
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