P0420

Discussion in 'Subaru Impreza' started by mjm4jc, Jan 26, 2019.

  1. mjm4jc

    mjm4jc

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    Hello everyone,
    I am new to the forum but not new to Subaru. Three out of my four daughters all have Imprezas (95 LX Wagon, 97 Sedan L, and 09 2.5i). The 09 we just bought this past June. Anyway, I wanted to share something with all of you about my 09 2.5i that has driven me nuts for about 6 months now. It has to do with P0420 code. And yes, I read just about every post on the internet about this code. From using my Scanner (with live data), I ruled out bad 02 sensors. From the way the car ran (like a beast, even up-hill) I pretty much ruled out a bad catalytic converter. I am not saying that the converter wasn't going bad or in the early stages of going bad which could throw a code. I just knew it wasn't "bad." I have had cars with bad converters before, and they almost always ran a bit sluggish before they've completely gone bad. This was not the case with mine.

    What I did notice after doing some maintenance on the car when we first purchased it was that the air filter box was missing the spring clips that hold the outer cover to the box. I didn't think anything of it at first, but then began thinking if it could have anything to do with the P0420 code. My thought was that if not enough air was being directed into the air filter, it could cause a rich condition. If the AF sensor/O2 sensors are working properly, then the computer may just throw a cat code. This was just an assumption. I lived with this code until just a few weeks ago, due to the fact that I could not find any spring clips for this car anywhere. There were some universal clips, but they didn't seem like a match from the pics on-line. I managed to find one clip that I had laying around from another Subaru and was able to secure the cover to the box pretty good. I then cleared the DTC, and it has not come back since. It has been at least three weeks now with no code. So, either this is a coincidence, or sealing up that cover fixed the problem. Keep in mind that when the cover is not fastened to the box, the intake is still sucking air like normal but the air duct is not directing the air into the box the way it should. At idle, I don't think that this is a big deal. But when driving on the highway, I believe that the intake relies on a direct flow of air from the air duct on top of the radiator into the air box.

    Please also keep in mind that what I am saying here is merely my opinion. I have no data to back it up. All I have at this point is a clear DTC that has not returned. I just wanted to share this with you. Thanks for reading. :)
     
    mjm4jc, Jan 26, 2019
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  2. mjm4jc

    Nate4x4

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    Good read, thanks for sharing! I'm happy you were able to solve your problem.
     
    Nate4x4, Jan 26, 2019
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  3. mjm4jc

    Nate4x4

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    Welcome to the site!
     
    Nate4x4, Jan 26, 2019
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  4. mjm4jc

    mjm4jc

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    Thank you!
     
    mjm4jc, Jan 26, 2019
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