Yet another P0420 question
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This is a discussion on Yet another P0420 question within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Greetings all. Posting on behalf of my GFs 2000 Impreza wagon with around 125K miles on it. Check engine light ...

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    Yet another P0420 question

    Greetings all. Posting on behalf of my GFs 2000 Impreza wagon with around 125K miles on it.

    Check engine light came on and Autozone says it's P0420 - Catalytic Converter Efficiency Below Threshold on bank one.

    It comes on every few weeks and I just disconnect the battery/beep horn/reconnect battery to clear. Every third or fourth time it comes back my girlfriend forces me to go BACK to Autozone to ensure it's the same code (which it always is).

    Unmodded, and stock exhaust as far as I can tell.

    Trying to explain to my girlfriend that this code is not dangerous to her or that the car is not two miles away from exploding has been a challenge, to say the least.

    I thought it was cat related till the most recent autozone guy said, "No, it's your front O2 sensor..see it says 'bank one'" but then I came here and I'm again convinced it's cat related.

    Either way, I'd like the code to go away. Car will be driven for another year, two at most, and I'm pretty sure it will pass PA inspection, but only vanquishing the code will provide me with respite from my girlfriend.

    Not above gettting my hands dirty and will fix the problem if it's under $150 or so, but if the fix is more expensive than that, then disabling the code is more my concern.

    So, all that being said, here are my questions.

    1) Front O2, cat, Rear O2..what's the most likely cuprit?
    2) How does one go about disabling this CEL?

    Thanks in advance!

    Mike

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    Registered User jd92677's Avatar
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    unfortunately, it the converter. The reason for the code is you have an O2 sensor before and after the converter to monitor it/them, you have more than one converter, hence the bank one. When the converter is working properly, the front O2 swings from lean to rich to help the ECU manage fuel/air mixture, while the rear stays pretty much in the middle, indicating that the excess "rich" is being burned properly. When the converter is not working properly, the rear O2 swings back and forth with the front one, indicating that the exhaust coming out of the converter is the same mixture as going in. The fix is not cheap because I believe on your car the converters are part of the exhaust manifold, which is on the expensive side. You won't pass inspection as long as that light is on and when you clear it, it clears the "rediness" monitor, meaning it hasn't passed its self test. You also won't pass inspection if the engine light is cleared and the monitors haven't passed yet so you really need to fix the problem in order to pass inspection. A little long for a response but I hope it helped, sorry the news wasn't better

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    JD, thanks much. If I have two cats, does that mean I have two downstream O2 sensors or just one?

    Either way, candidly speaking, replacing it is not in the cards since we're only going to have the car 12-18 more months. Would an O2 spacer do the trick?

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    Registered User jd92677's Avatar
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    Quote Originally Posted by mikestorm View Post
    JD, thanks much. If I have two cats, does that mean I have two downstream O2 sensors or just one?

    Either way, candidly speaking, replacing it is not in the cards since we're only going to have the car 12-18 more months. Would an O2 spacer do the trick?
    Being that the code was a bank 1 code, I would assume that there is a downstream O2 for each converter to monitor them separately. You are correct that there is no driveability concern, your' wife will be perfectly safe driving the car but you probably won't pass inspection unless you replace the converter

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    Well, I paid $5 for a pair of spark plug non-foulers and another $4 for an O2 socket.

    I reamed out one of the non-foulers per the instructions, but unfortunately there wasn't enough clearance to install both stacked on top of the downstream sensor per the web instructions, so I only installed one (the reamed one obviously).

    Still, the sensor is a bit more pulled back and a bit more shrouded, so hopefully the mechanical fix will decrease the code tripping if not addressing the issue entirely.

    JD, thanks again for your advice, but as I said, replacing the cat isn't an option.

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