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2003 Nissan Sentra SE-R Spec V 2.5L QR25DE
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I have a 2003 Nissan Sentra SE-R Spec V. I currently have been having these issues and wanted to here if y'all had any ideas. Okay so I had only the code P0420 the check engine light disappeared for a bit then randomly came back on but this time with code P0302 I originally fixed code P0302 with new spark plugs and now it's back for the code P0420 I just had my car at the dealership shop which they changes both O2 sensors so it's not that also for some information a few days ago I quite literally had everything after the precat fall off my car when driving so I only have the exhaust manifold and precat both O2 sensors are there. My question 1st question is for the P0420 should I just dump out the cat until I can get an aftermarket exhaust manifold or is there something I should try before that? My 2nd question is after I check the spark plugs if there still good should I chang the coil pack and spark plugs boots and go from there?
 

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You could have a intake leak some where after the MAF leaking unmetered air into the system, the other is there may be a leak around the exhaust manifold at the head or or from a hole some where on the manifold. FSM also suggests a timing issue but I highly doubt it, I could see a coil pack causing it due to the misfire issue.
 

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2005 Sentra SE-R spec-v
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I have a 2003 Nissan Sentra SE-R Spec V. I currently have been having these issues and wanted to here if y'all had any ideas. Okay so I had only the code P0420 the check engine light disappeared for a bit then randomly came back on but this time with code P0302 I originally fixed code P0302 with new spark plugs and now it's back for the code P0420 I just had my car at the dealership shop which they changes both O2 sensors so it's not that also for some information a few days ago I quite literally had everything after the precat fall off my car when driving so I only have the exhaust manifold and precat both O2 sensors are there. My question 1st question is for the P0420 should I just dump out the cat until I can get an aftermarket exhaust manifold or is there something I should try before that? My 2nd question is after I check the spark plugs if there still good should I chang the coil pack and spark plugs boots and go from there?
Since the cats are a known catastrophic failure point, eliminating that potential cause of failure would be a good idea in my opinion. Your misfire will just increase the likelihood of a failure.

The misfire can also cause strange O2 readings, so resolving that might resolve your cat efficiency fault as well. If this wasn't a car where the cat is a ticking time bomb, I would advise you to fix the misfire first. For diaging the cylinder 2 misfire, swap coils with another cylinder. Swap the spark plug with a different cylinder. See if the misfire follows either one. You could do the same with a fuel injector, but the way the fuel rail is buried might make it easier to check compression and cylinder leak down first. (Especially since cat failure is possible causing physical engine damage.) If everything else checks out, and the mis stayed on number while swapping the coil and plug, I would probably just replace the injector.
 
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