# P0420: Catalytic Converter or oxygen (02) sensor?



## klwdallas (Jan 20, 2008)

2000 Sentra GXE (1.8)

Had to drive this car for 500 miles with one cylinder misfiring (weekend, in small town, baby in back, etc.). I also poured some 'additives' in the gas hoping it was bad gas since the locals said this was common.

Fixed it after scanning OBD2 revealed a bad coil.

Now I'm getting a P0420: I am pretty sure my cat is bad now (unburned gas + additives into cat), but I want to verify before spending ~$800 dollars (unless I do it myself).

I've found some good posts for monitoring the sensor activity and I do have access to an oscope.

OXYGEN SENSORS
Troubleshooting P0420 Catalyst Code

I read about upstream and downstream sensors, and bank one and bank two. My question is what are the 4 sensors for in the engine compartment? Are two of these upstream and two downstream? I don't think so because the downstream one should be past the cat.

Next question, where is the connector for the downstream cat? I haven't jacked the car up, but I couldn't see it from underneath. Is there a better place (in the engine compartment) where I can monitor this sensor?

Thanks.

P.S. Why does the computer not stop sending gas to the cylinder when the coil is not causing a spark?


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## klwdallas (Jan 20, 2008)

Nothing? Are there any other forums?


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## klwdallas (Jan 20, 2008)

*Got the shop manual*

Got the Nissan shop manual

This car is not California type so it has two cats in engine compartment only, none underneath. All oxygen sensors are also in engine bay.

Looking at likely causes and given how quickly problem started, this is likely one of the catalytic converters. Shop manual does not even consider the 02 sensor a possible problem, which is contrary to what I've read in other places.

For now I replaced the spark plugs with the ones called for by manual instead of the cheapies from Autozone, which I had installed at same time as coil. I then reset the light with my obd2 tool. I'm pretty sure it will come back, in which case I'll monitor the rear O2 sensor for the bank in question with an o-scope. If it looks ok, I'll replace the cat.


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## enzorug (Oct 13, 2010)

What was the final solution to your problem? I have had the exact same experience with a Sentra GXE 1.8 at 92K miles. The car started to run really bad with a misfiring piston. Took it to a shop here in VA called Merchants and they told me it was one of the ignition coils. The replaced it with also 4 new spark plugs, reset the Service Engine light but it came back on a few hours after I got it back. I reset the light which was giving the same error code but it came back a week later. I don't know what to do and prices to replace the cat are more expensive than the car is worth. Any idea? Do I need new spark plugs maybe?


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