Q: P0420 bad under body catalytic converter 2003 Nissan Altima

asked by on February 13, 2017

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.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

rattling noise when engine idles. load engine noise during acceleration like glass packed muffler

This may be related to loose exhaust parts such as exhaust mounting brackets, or potentially a bad or failing catalytic converter. As you may know, the catalytic converter has a honeycomb-like structure inside of it which helps to convert toxic...

hello I have an Infiniti FX35 2003 and replaced water pump thermostat and radiator. Car still overheating.

Hello. I would need to know when it is overheating in order to be able to diagnose what is going on. I would start though by making sure that all of the air is out of the cooling system by...

Where is the neutral safety switch on a 2003 Cadillac CTS with an automatic transmission.

On your vehicle, the neutral safety switch (also known as the "manual shift shaft position switch") is inside the transmission case. To access the switch the transmission has to be lowered (slightly) and there is some disassembly required to do...

Related articles

2012 Nissan Sentra vs. 2012 Nissan Altima: Which One Should I Buy?
When you consider the Nissan Sentra versus the Nissan Altima, you need to realize that you’re crossing between two different classes of vehicle: the Altima is a mid-size sedan while the Sentra is a compact sedan. The differences between the...
Symptoms of a Bad or Failing Catalytic Converter
Common signs include reduced engine performance, rattling sounds, and the Check Engine Light turning on.
A Buyer’s Guide to the 2012 Nissan Frontier
When When looking for a truck, the 2012 Nissan Frontier could be the vehicle you’ve been hunting for. It has a great look, a strong engine, and it is reliable. The vehicle has been making gains on other trucks in...