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Bad O2 sensor or bad cat?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Prius_let_me_go, Sep 11, 2024 at 1:47 PM.

  1. Prius_let_me_go

    Prius_let_me_go Junior Member

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    Vehicle:
    2004 Prius
    Model:
    II
    Is there any good way to see if my O2 sensor is what is setting my check engine code or if it is just a bad (or missing) catalytic converter? I bought the car very used in June of 2022 and when I bought it I noticed some exhaust modification had been done and have been driving it since with the P0420 code on and figured the cat was removed or just clogged (I also just checked it and I now have a P0171 and P1121 code which I will deal with separately). I have seen other posts about looking at the fuel trim or voltages from the sensor, but nothing specific. Also I feel like it's the cat because of the bad smell from the exhaust. I also am burning a little bit of oil, and with that being the case I feel like even replacing the cat is not worth it at this point because it will likely get clogged if it is replaced. (I also don't think replacing my PCV valve will do much good at this point either).
     
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    2006 Prius
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    Four
    Deal with the P0171 lean run condition first. The catalytic converter (P0420) test is only valid if the engine is running correctly.

    If the P0420 has been around for years, it may be that it will need a new cat anyway, but you have to fix the fuel trim problem 1st.



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    Brian1954 and Danno5060 like this.