I have a 4 cyl 2002 Nissan Altima that is giving a trouble code that says I my catalytic converter is bad. This car has 2 catalytic converters in line. How do I know which one to replace?


It is the rearward oxygen sensor that is triggering the code…I would replace that part first…It monitors the performance of the converter. It is more prone to failure than the converter itself…


Well, I replaced the downstream O2 sensor on Saturday. This light is still on. I think I’ve given it enough time to turn off, I’ve been driving it back and forth from work. I took it to AutoZone and had the codes read. The guy at AutoZone said that its the P0420 code and replacing the O2 sensor wouldn’t fix the problem. He said instead it would be a fuel system problem or in fact a failed catalytic converter. So, do I need to replace the front catalytic converter?


I don’t have a Nissan Altima to look at, but the picture of the parts appears that the rear converter is downstream from the second O2 sensor. If this is the case, the computer is only monitoring the front converter and has no way of knowing what the rear converter is doing. It is saying the front converter is bad. I only assume the second converter is to super-clean the air since it has what looks like an air injection hose to it.

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You can and should verify but 420 is bank one and 430 is bank 2. 420 was the rear cat on mine and a new cat took care of it. One bank and one cat for each side of the engine.


That Guy At Auto Zone Is Giving You Some Good Advice.This Is A Very Common Occurrence With Cars From All Manufacturers.

I wouldn’t go replacing any converters, either. Chance are that won’t eliminate your DTC. The code you’re getting is one of the most common codes that trigger a “Check Engine” light and the causes are many.

Has anybody turned off the “Check Engine” light ? That’s what I’d do, first. See if it comes back on or how long it takes. Also, look for other codes to appear that could provide more clues.

I’d even consider spending 50 bucks (at Sears, I believe, but available at auto parts stores, as well as other brands) and becoming the proud owner of my very own code reader (like an Actron Pocket Scan). It will give you codes pending, codes set, and allow you to turn off the light. An included booklet translates the codes.

Any driveability problems, rough running, any symptoms ? Does the car make a lot of short trips ? Have you changed driving patterns ?

Turn the light off and see what happens. Here’s 2 links to some information. Let me add that the causes for PO420 go beyond the short list given there. Sometimes the cause(s) won’t trigger a DTC of their own that would help you. OBD-Codes.com

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P0420 Catalyst System Efficiency Below Threshold

How to diagnose, repair, and info on symptoms and causes of a P0420 engine code which means the catalytic converter is no longer performing at a normal level.


Unfortunately, you have the very unpleasant P0420 code on an "02 Nissan Altima.I would highly recommend that you do a Google search, and find out not only how common and widespread this issue is, but how potentially damaging it is to the engine.

Usually, by the time you get the CEL, the loose pieces of catalyst have already done damage to the cylinder walls, and you have oil consumption issues for the rest of the vehicles lifespan.

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If you have oil consumption issues already, you might as well start saving up for a new engine. If you don’t have oil consumption issues, replace the convertor immediately. Its the one that’s built into the exhaust header, that bolts directly to the engine. If you’re lucky, you won’t have oil consumption issues afterwards, and the new convertor won’t fall apart on you like the original one did.