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New Prius owner needs help with P0420

Discussion in 'Gen 2 Prius Main Forum' started by Moonlitsouls, Oct 20, 2021.

  1. Moonlitsouls

    Moonlitsouls New Member

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    2005 Prius
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    ----USA----
    Hope all is well, I just picked up an 05 Prius with 145k and I love it

    The day I bought it the CEL came up for P0420 for the bad catalytic converter

    I also had a bad small of exhaust leaking into the cabin

    I changed the cat to one I found on eBay from a reputable shop in Canada that people recommended on this forum, cleared the code. the exhaust smell went away but the code came back

    I then changed the Downstream O2 sensor , cleared the code and the code came back again

    The car is running noticeable better with the new cat , but I’d appreciate any tips to get rid of the CEL, I saw some people adding a spacer to the o2 sensor but I’m confused As to why/how the code is coming back if I installed a new cat and sensor

    would greatly appreciate the help

    thanks
    Adam
     
    #1 Moonlitsouls, Oct 20, 2021
  2. SFO

    SFO Senior Member

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    Welcome to PriusChat!!

    Did you use OEM / Denso sensor(s)?

    FYI : you're being moderated until you've posted 5 times.

    edit : typo
     
    #2 SFO, Oct 21, 2021
    Last edited: Oct 21, 2021
  3. koco

    koco Member

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    The previous owner probably cleared the P0420 code right before you bought it. The exhaust probably had a leak that changed the levels detected by the 02 sensor and triggered a code.

    Have you cleared the codes since the new cat install?
     
    #3 koco, Oct 21, 2021
  4. Moonlitsouls

    Moonlitsouls New Member

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    hey thanks, yea I did use OEM sensor

    hello, yes I did

    thanks for the replies, thats why I’m so confused by this
     
    #4 Moonlitsouls, Oct 21, 2021
  5. mr_guy_mann

    mr_guy_mann Senior Member

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    Four
    When I diagnose a P0420 cat code I go through a checklist. 1) NO exhaust leaks from the exhaust system from the manifold gasket back to half a meter past the downstream O2 sensor. Not even a pinhole. 2) No TSB's or other dealer software fixes for the problem. 3) I use a scantool (such as Techstream) to verify proper engine and Air-Fuel / Oxygen sensor operation (no misfires; total fuel trim or AF sensor feedback is 0 +/- 10%; I use a special test on the scantool to force both AF & O2 sensors full rich then lean to check the sensors and catalyst operation).

    IF everything checks out THEN I replace the cat. Cheap units are just that- cheap. Might work ok, might trip the check engine light next day, or next month, or next year. Who knows?

    This is a complex system and takes some savvy and tooling to properly diagnose. Maybe you have a problem with the upstream AF sensor or basic engine operation. Without proper testing you can keep replacing parts and hope that it keeps the light out.

    Depending on how personally involved you want to get in figuring this out, you should find a copy of the service manual and look at the diagnostic routine for that code.

    Posted via the PriusChat mobile app.
     
    #5 mr_guy_mann, Oct 22, 2021
    Moonlitsouls likes this.
  6. Moonlitsouls

    Moonlitsouls New Member

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    thanks a lot for your reply , well i guess im stuck with it for now. at least the exhaust smell went away and the car is driving well
     
    #6 Moonlitsouls, Oct 24, 2021
    SFO likes this.
  7. MilkyWay

    MilkyWay Active Member

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    Location:
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    Vehicle:
    2007 Prius
    Model:
    II
    I've had a P0456 for the last 140k miles. "Very small evap leak". Doesn't effect anything for me in the last 5 years / 140k miles driven.

    Used to clear the code it would stay off for weeks at a time. Eventually stopped caring. Has 280k-ish miles.
     
    #7 MilkyWay, Oct 26, 2021
  8. mr_guy_mann

    mr_guy_mann Senior Member

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    Four
    You can ignore the check engine light if you want to- but some people live in areas where they have inspections. So they need to (occasionally) have the light stay off long enough that the car can complete most of its monitor tests and pass.

    Posted via the PriusChat mobile app.
     
    #8 mr_guy_mann, Oct 26, 2021
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