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OK guys, I have an 06 Titan 4wd with 180,000 miles that is 100% stock. I changed the spark plugs about 3 weeks ago and after driving for about 200 miles the SES light came on. I pulled the codes and got only the P0430. So I reset the light and decided to put some mid grade 89 octane in her. She has been running great and my mileage is a little better since I changed my plugs. They are Denso Irdium btw.

So I drive for another week and put another 700 miles or so switching back to the 87 octane. Now the light comes back on last Tuesday. It ran that way for a week and now I pull P0420 and P0430 codes. So today I put some premium (93) in her and the SES goes off again. Anybody have a clue why this is happening? What would you do? Thanks for your thoughts!
 

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Interesting. The dreaded P0420 code is Catalytic Converter efficiency below threshold. Its possible when you were running 89 octane that it burns cleaner therefore the 02 sensors are not picking up that the cats are going bad because of the cleaner burn to start off with? That is if it didnt come back on until you ran 87 again. Just a thought... someone will prob chime in here.

I ended up having to replace my exhaust manifolds in fear of sucking cat material into the engine because of Nissans valve overlap for EGR (exhaust gas recirculation). Keeps the exhaust valve open just a bit during intake stroke.
 
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