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P0420 Permanent Code

Discussion in 'Gen 3 Prius Technical Discussion' started by donzoh1, Mar 3, 2023.

  1. donzoh1

    donzoh1 Active Member

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    Code P0420 is causing check engine light. After clearing the code, it is immediately pending again and will return to permanent within 20 or 30 miles. I've measured back pressure and it's minimal at idle and around 1 PSI at 2500 RPMs so I don't think there's any restriction. I do believe this car has burned quite a bit of oil which may have fouled the catalytic converter. My question is whether this code would be immediately pending after clearing simply due to a fouled cat or does this behavior indicate either a wiring or sensor problem. Wouldn't the fouled cat take at least a few seconds to show a pending problem? I'm assuming the pending code goes away when codes are cleared.
     
  2. ChapmanF

    ChapmanF Senior Member

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    P0420 reports a failing result of the test the car runs for the catalytic converter's oxygen storage capacity (OSC).

    There are more than just 'pending' and 'permanent' engine DTC states. There's also 'current'. P0420 will be pending after the test fails once, and current (and also permanent) after the test fails twice.

    You can clear it from 'current', but the 'permanent' stays until the test has passed in three consecutive drives. (It can be hurried along by driving the confirmation pattern for that code in the repair manual.)

    Interesting about it coming back 'pending' right away; I hadn't noticed that before. Might be because the permanent code is still there, so they don't feel like waiting for two tests to fail before saying the problem's still not fixed.
     
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  3. nicoj36

    nicoj36 Active Member

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    Try this catalytic cleaner. There's reviews on it that has the same issue as you (p0420). Also clean your MAF for better airflow and that will help your exhaust system have better flow.

    https://a.co/d/9uy5FYb
     
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  4. ChapmanF

    ChapmanF Senior Member

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    Having a clean mass airflow sensor is never a bad thing. It helps the car know what the airflow is.

    At the same time, saying that will give you better airflow, and that will help your exhaust system, to have better flow, and that will help the catalytic converter, to pass an OSC test, that it is currently failing, gets more and more farfetched, at every comma.
     
  5. nicoj36

    nicoj36 Active Member

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    The better air-intake a clean MAF provides, the more output the exhaust emits. Which will help clean his catalytic along with the cleaner.
     
  6. ChapmanF

    ChapmanF Senior Member

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    Still trending the same direction
     
  7. donzoh1

    donzoh1 Active Member

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    On my scanner, it says p0420 1/1 Permanent but the yellow light is on indicating Pending. After some driving (maybe one drive cycle) the scanner light turns Red and the CEL comes on.
     
  8. PriusCamper

    PriusCamper Senior Member

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    You are posting this question in the forum for 2010-2015 Prius, yet your profile info indicates you have a 2004 Prius? The recommendations for a 2004 Prius with this error code will be way different than a 2010-2015 Prius. Please clarify?

    Additionally, your options will be different depending on what state in the US you live in... Or maybe you aren't in the US? If you lived in California you'd be looking at a way more expensive challenge than many other states. Please clarify?
     
  9. PriusCamper

    PriusCamper Senior Member

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    Maybe removing the cat and soaking it in this stuff would be useful... Is that what the directions suggest? Or is it applied in some other less effective way?
     
  10. donzoh1

    donzoh1 Active Member

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    It's a 2013 Prius which I should have clarified. I also have a 2007 but not sure where the 2004 came from.
    It's a 2013 although I also have a 2007. I tried to modify my profile to 2013 but was not able to so maybe a mod could do that. Anyway, I'm in Kansas so no smog issues. About 100 miles ago I put one can of SeaFoam intake/carb aerosol through the air intake and today I put a can through the first O2 port. I'm still not positive whether the P0420 is from a failed sensor or from oil fouling but that's definitely a possibility. It has 170K and this is the original cat and the original sensors. If I replace stuff, it will not be with new OEM since that would be 25% of the car's value. I might go for an O2 sensor spacer just to eliminate the CEL.
     
  11. PriusCamper

    PriusCamper Senior Member

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    Thanks for clarification... How many miles on it?

    If it were my car before I add the spacer to the sensor I'd first clean the EGR, the throttle body, intake manifold, MAF and add new spark plugs and air filter, reset engine settings by disconnecting the 12v. If that didn't work I'd soak/clean the cat, then try a new sensor. Once I did all that in that order if it came back I'd resort to the cheat device on the O2 sensor with long term plans to replace the cat sooner than later.
     
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  12. AzusaPrius

    AzusaPrius Senior Member

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    Try Cataclean
     
  13. PriusCamper

    PriusCamper Senior Member

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    Is that something you physically soak a cat in or some kind of pre-combustion pour and pray scam?
     
  14. AzusaPrius

    AzusaPrius Senior Member

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    You pour it in your gas tank.

    This is not a scam product and it works.

    If you know the story of my Prius then you would know that I have had issues with P0420 and replaced my cat because my engine had a bad HG and destroyed my OEM cat.

    I used this product and exactly 30 days later my code went away.

    My cat was really damaged so the code came back and then off/on until I replaced it but before I used Cataclean, the code was on for a whole year without ever going away.
     
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  15. PriusCamper

    PriusCamper Senior Member

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    Ok, I got a a friend with a never ending 420 code that finally came back 6 months after we replaced the Cat after exhausting all the other options other than changing the sensor. So we'll try Cataclean next because I gotta pop all the rivets on the cat shield to swap the sensor and I used oversize rivets, so...
     
  16. donzoh1

    donzoh1 Active Member

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    Cat removal and soak would be the next logical step but it has no detectable exhaust flow restriction at this point. I did order an O2 spacer so will see how that goes.
     
  17. donzoh1

    donzoh1 Active Member

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    Cataclean was tried with no noticeable benefit. I think this cat has been pushed way past that point, but maybe 30K or so miles ago would have worked.
     
  18. donzoh1

    donzoh1 Active Member

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    The SeaFoam can is normally recommended for spraying directly into the intake airstream with motor running. When can is empty, the motor is immediately turned off for heat soak of intake valves, etc. It also cleans carb and (where equipped) and other components. Spraying into the first O2 port is NOT an official SeaFoam suggestion, I don't believe. I did on a cold engine, which might have been more effective hot?
     
  19. donzoh1

    donzoh1 Active Member

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    I've read some cases where the pour in stuff worked but I'm guessing this would be VERY EARLY in the process. If there is continued contamination, I doubt they'd work.
     
  20. donzoh1

    donzoh1 Active Member

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    I'm guessing you know this already but the EGR cooler (dirty/restricted) is a likely root cause which will eventually destroy the Cat or do even worse. The EGR cooler was VERY CLOGGED on this Gen 3 before I got my hands on it. Before this, it also caused various cylinder misfires, a broken head gasket which cascaded to a bent piston rod. Beyond this, an oil catch can might prevent Cat contamination and various other issues. I just installed one and am going to add a drain valve and tube to it.