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Discussion Starter · #1 ·
I got the P0420 code the other day. 2008 Altima 2.5L. I replaced the manifold/cat about 3 years and 50,000 miles ago because the dealership said that it was cracked. (Not sure if I believed them but I replaced it anyway because I could not test the manifold). I got one from Rockauto. I have a OBD II scanner and when I look at the rear O2 sensor it is a flatline at 280mV. I would think that it should be 500mV if the cat is working correctly? right? but even if it is at 280mV it is not going up and down like the A/F mixture is changing which means the cat is working. What is weird is I can't find a voltage for the front sensor on my scan tool. I would like to see the voltage reading for that. So my question is, would this just be a bad rear sensor? because the voltage is not changing?

Anyone have experience with this?

Thanks in advance.
 

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Altimer
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P0420 is a significant issue on the 2.5. It is overwhelmingly a converter issue. I would see if there is any warranty left on the converter you purchased from rock auto.
 

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Discussion Starter · #4 ·
I will see what they say. Thanks for the reply. I was a little leery when the dealership told me that the original manifold/cat was cracked. I had an exhaust manifold gasket replaced under warranty 5 years ago at 65,000 miles. Then three years ago at 133,000 miles there was another exhaust leak sound so I took it back to the dealership and then they said that the manifold was cracked and needed to be replaced. I ordered my own manifold/cat and replaced it. I thought it was another exhaust gasket leak. I still have the original manifold/cat, I thought about getting a new gasket and putting that one back on. I have seen some youtube videos about cracked manifold/cats on these Altimas. I couldn't see any cracks or any markings where there could be a crack on my original cat.

Thanks.
 
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