02 Nissan Sentra GXE DTC P0420 & P0430

George

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2002 Nissan Sentra GXE with 1.8L 49-state engine, 88K miles. Bought it new and plan to keep it til the wheels fall off, and have faithfully maintained it (over-maintained probably). It has always been dependable and trouble-free.

A little whle back while driving, it suddenly started bucking, hesitating, popping, backfiring. The MIL came on. The idle was rough and varied wildly on its own. Finally it died on the side of the road and would not restart. This was so bizarre for this car that I had it towed to the Nissan dealer.

The told me the fuel was contaminated so I had them clean the entire system from the fuel tank forward. All was well until recently when the MIL came on. This time I used my scantool and found codes P0420 and P0430, indicating a malfunctioning cat converter. I know that a DTC does not necessarily point to a defective part but is rather a starting point for diagnosis. I've always beleived in proper diagnosis and repair instead of just hanging parts till the problem goes away, so I dug in.

The first thing I checked was the O2 sensors. Both upstream sensors seem fine, good crosscounts from lean to rich and good switching frequency at idle and 2000 rpm. However, both downstream sensors mmic the behavior of the upstreams. These sensors should read mostly lean and not switch much, but their readings were very similar to the upstreams.

The only other problem I found besides these codes and the downstream O2 sensor readings is that when I pulled the plugs to examine them, all four plugs were wet with fuel on the tips. Did a fuel pressure test, and it was within specs with and without vacuum. FP regulator also passed. Intake manifold vacuum was around 19 at idle; rev it up and it goes to the low 20s then slowly drops, which tells me there are no intake leaks and the exhaust is not restricted. Checked carefully and found no exhaust leaks. Inside of tailpipe is clean, no black soot or anything else, and no smoke of any color when running. Also pulled the fuel rail and checked for leaking injectors, but they appear good.

All other readings from my scantool were within specs: timing, MAF output, idle speed, load value, airflow, etc. My tool does not show injector pulse width, but short term fuel trim averages around -4.7%.

The car runs perfectly, no driveability symptoms at all. Smooth idle, good acceleration, starts quick, and all this under cold and hot conditions. If not for the MIL being on (and 4 fuel-soaked plugs) you would never suspect anything was wrong -- and I'm quick to notice when something is not quite right. There are no other DTCs besides the 0420 and 0430.

The main thing that's throwing me is the plugs. If I really did have bad gas, I suppose that could ruin the cat, but why would I have fuel-wet plugs when everything else seems fine? I also don't know if the downstream O2 sensors are contaminated from bad gas or just responding as they are because maybe the cat is gone. And the front O2 sensors seem normal anyway. I also wonder if maybe the PCM is bad -- holding the injectors open too long, misinterpreting O2 sensor readings, or both.

I'm looking at an expensive fix here and don't want to replace something that doesn't need replacing. Is there a test or a check that I've missed that might expose the offending part? Have I misinterpreted the results of any of my tests? If the cat is bad, what does that have to do with fuel on the plugs? I'm even wondering if I did have bad gas to start with; as we all know even dealer shops and the best mechanics miss a dagnosis on occasion.

I would greatly appreciate some advice and opinions on what is causing these problems and how to get them fixed. I pride myself on being a fairly competent weekend mechanic but this one has me scratching my head :)

Thanks a lot!
George
 

Paul

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How long does the fuel psi hold after the engine is shut off?
 

George

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After shutdown it dropped a few psi then held steady for 10 minutes, that's when i disconnected the tester.
 

2POINTautO

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Little bit of light reading to understand the code:
http://www.troublecodes.net/articles/catfailure/

Ford_Dude just made a reference on another post recently, it seems you are an eperienced guy.

If the cat doesnt light off, it is not going to work, drive it hard and get the exhaust hot.
 
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