Question: I have a 2001 Nissan Pathfinder with the 3.5-liter V-6. The check-engine light came on, and I had code P0420. I was told it was a bad catalytic converter, so I went ahead and replaced it myself. I cleared the code and a week later the light came back on with the same code.

Answer: You have two catalytic converters on the Nissan Pathfinder 3.5-liter, one for each three-cylinder bank of the engine. Typically engine bank 1 is the side including cylinder #1. In your case, it’s the right side of the truck while sitting in the driver’s seat. You’ll see what I’m getting at in a minute. P0420 code represents “Catalyst System Efficiency Below Threshold, Bank 1.” It is usually, but not always, caused by a bad converter that’s not reducing emissions output as it should. This is determined by the PCM while comparing the readings of the oxygen sensors located before and after the cat. First and foremost, be sure you replaced the converter on the right side (Bank 1). It may sound funny, but this mistake occurs more often than you might think. P0430 is the DTC for Bank 2 (left side). Aside from the converter itself, inaccurate readings from the O2 sensors, exhaust leaks, aftermarket converters, and add-on performance exhaust and intake parts can produce the same diagnostic trouble code(s). I suggest bringing your vehicle into a Nissan service department. They’ll probably charge you about $100 diagnostic fee. But there is a technical service bulletin involving incorrect DTCs P0420/P0430 that can be corrected with PCM software reprogramming.


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