# just bought a 2008 sentra have p0240 code



## miran587 (Mar 30, 2015)

i have a 2008 sentra sl 2.0 and ce light is on giving code p0240 anyone tell me id its cats or o2 sensors car has 147k. i noticed rubber intake tube has few tears in it so i ordered new one but wrapped up in tape good till it comes in not sure if from it running with rip it was taking lot of air causing o2 running wrong from unmetered air


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## smj999smj (Jan 1, 2006)

I think you mean P0420, which is a three way catalyst code. Most of the time, if set without oxygen sensor or other codes, it's usually due to an inefficient catalytic converter. But, since you have a torn intake duct, I would replace the intake duct, first, erase the codes, and then see if the P0420 code returns before doing anything else.


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## miran587 (Mar 30, 2015)

Yes sorry about wrong code u are correct. If it comes back is it definitely cat or can it be o2 sensors is there way of telling


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## smj999smj (Jan 1, 2006)

As I mentioned, usually faulty O2 sensors will set a code. If you want to check the operation, you'll need a scan tool to graph the O2 sensor signals while driving at highway speeds (best done with two people for safety reasons). On each bank, the frequency of cycles going from rich to lean of the front sensors should be about twice that of their respective rear sensors on the same bank. This is the way that the ECM knows that the upstream catalytic converters are working efficiently. If the ratio is close to 1:1, then the converters are not doing their job at breaking down emissions and require replacement. This assumes that all other possibilities have been ruled out, such as exhaust leaks. If you want to properly diagnose the P0420 code, find a free download of the "EC" chapter of the factory service manual for your vehicle and follow the diagnostic procedure for DTC P0420. You can sometimes find this in the "knowledge base" at NissanHelp.com and also downloadable manuals at the Nico Club's website (usually in .rat format, which require WinZip or a similar program to extract the files).


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