# code P0420



## kawboy636 (10 mo ago)

I have a 2009 Sentra 2.0 that kicked out a P0420 code. The car starts and runs fine with no adverse symptoms. A few months ago I replaced the valve cover gaskets and spark plugs because the #4 spark plug tube seal was leaking oil and it tripped a misfire code. I caught that problem very early and corrected it. The car has been driven several hundred miles since then with no issues. Does anyone have experience with this code on this gen Sentra?


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## VStar650CL (Nov 12, 2020)

P0420 is a "dying cat" code, and it may be that your recent misfire damaged the catalytic converter. When dealing with it, you need to recognize that low efficiency is one thing, failure of the honeycomb is another. With a cat that's losing efficiency, a treatment with Cataclean or a similar product, or installing spacers on the rear O2 sensors, will often get rid of the code. However, you're masking the problem by doing so, and you shouldn't try stuff like that if the cat is actually dead. Nissan/Infiniti engines are practically all Miller-cycle and will inhale the debris if the honeycomb melts, with catastrophic results. "Catastrophic" means good-bye engine 100% of the time, so it isn't something to trifle with. Regardless of what the sensors read, an IR thermometer can tell you how bad it is very quickly. Get the front cat lit on a decent drive and then measure the temp at the inlet and outlet just past the pipe welds. The outlet should be substantially hotter than the inlet, 100~130F on a healthy one, probably 30F even on a marginal one. If it's much lower than that, especially if the outlet is equal or cooler than the inlet, you're risking your engine by screwing around with half measures.


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## VStar650CL (Nov 12, 2020)

These are a couple images from another website of a cylinder on a QR25 Altima that inhaled a melted cat. What you're looking at is 2000F+ debris welded to the valves, piston, and cylinder walls. This engine is completely unsalvageable, and that's the result of _all_ cat inhalations. 

Maybe you get lucky and the honeycomb clogs or breaks away instead of melting, but as they used to say in older times, "You pay your two bits and take your chances." 'Nuff said:


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