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Q: P0420 bad under body catalytic converter 2003 Nissan Altima

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Hello, I have my car that had many issues that I was able to repair myself. For example, fouled spark plugs and ignition coil packs because of a bad valve cover. I replaced it and car is running well. Then I get code p0420, I sometimes get excessive smoke from tail pipe during some cold day startups. I took to mechanic for diagnostics to find which catalytic converter is bad. From my understand my car has two of them, one accessible from the engine bay and the other under the car. If I remember right the mechanic found the rear one has gone bad. I will call to make sure it's that one. But for my question, why would I get that code if there's no O2 sensors after that specific catalytic converter. Am I wrong about not having O2 sensors after the last converter on my car? I sure hope I'm wrong because that would mean it's the one In the engine bay and not the one under the car. From what I understand I have two O2 sensors in the engine bay only. Please help and thank you in advance :)

My car has 145000 miles.
My car has an automatic transmission.

Hi there - you are correct that there are 2 catalytic converters on your vehicle; one (sometimes called a pre-catalyst) that is part of the exhaust manifold, and a 2nd converter further "downstream" in the exhuast system. It is the 2nd converter that has oxygen sensors before and after, and usually triggers an "under-efficiency" issue. There is, however, a Nissan Recall (Recall 03V084000) on the "pre-converter" that is part of the exhaust manifold, and may be replaced under that Recall. If you are unsure if this repair has been done, check with your Nissan dealer. If the 2nd converter does need to be replaced, I recommend a catalytic converter replacement performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.

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