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Need some help.

I bought a 2007 STS v6 recently and the car was misfiring and throwing misfire codes and P0420 and P0430. I changed the spark plugs/air filter/ cleaned the MAF and now the car runs great, but the P0420 and P0430 code still exists. All the misfire codes run away and my scanner no longer defects any misfires.

I also noticed that the car seems to be running very rich. The exhaust is making smoke and smells like fuel. It runs absolutely fine with the exception of sometimes not starting on the first push of the button. I’m averaging 12 mpg with city and highway driving and from what I’ve read the car should get at least 16-17. The LTFT on both banks is about 11. It’s not throwing any other codes though. The upstream oxygen sensors are reading pretty much steadily 0v (-.03- .03v). I pulled the downstream oxygen sensors and they were covered in black soot. The spark plugs were also covered in the same black soot.

What kind of issues could be causing this? Would fouled upper oxygen sensors be a potential culprit? I haven’t pulled them out yet because of where they’re located. I’m thinking the P0420/30 code might not necessarily be the cat but just the car running incorrectly.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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2008 STS Northstar RWD, 2000 Trans Am WS6 6 spd, 1968 LeMans convertible 6.6
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It could be lean biased upstream O2 sensors. Since you car is not misfiring anymore, running smoothly, that would be where I would look first, and probably replace those upstream O2 sensors. Your fuel trim readings look good, usually anything +/- 10% is ok. If you had a leaky FPR or leaky injector(s) your trims would be massively negative, not positive.

P0420/430 is almost always a catalyst efficiency issue, and the PCM will check the O2 sensors for proper response before it runs the catalyst efficiency test. So, no, those codes are not bc of it running incorrectly.

Believe me, I have over 30 years experience working on cars and have seen many biased O2 sensors causing cars to run rich. It could also be other things! Like a biased MAF.
 
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