Bad O2 sensor or bad cat?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Prius_let_me_go, Sep 11, 2024.

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  1. Prius_let_me_go

    Prius_let_me_go Junior Member

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    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).
     
    #1 Prius_let_me_go, Sep 11, 2024
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    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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    #2 mr_guy_mann, Sep 12, 2024
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  3. Carall

    Carall Member

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    If you smell exhaust gases, the P0171 code is most likely related to the exhaust system. The fuel trim will show a lean mixture while driving.
     
    #3 Carall, Sep 12, 2024
    Last edited: Sep 12, 2024
  4. Prius_let_me_go

    Prius_let_me_go Junior Member

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    I changed the O2 sensor and the code went away but I still have the lean fuel code, I cleaned off the very dirty MAF but that only kept the code from coming on for about an hour of driving on the highway and then when I restarted it it popped back up.
     
    #4 Prius_let_me_go, Oct 8, 2024
  5. Danno5060

    Danno5060 Active Member

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    Take a look at the short and long term fuel trims after the engine warms up. They'll probably tell you your car is trying to increase the mixture.

    Maybe check for a vacuum leak. Put it in maintenance mode so the engine runs. If it's a vacuum leak, the fuel trims (short plus long) will be be high at idle, but trend toward zero if you rev up the engine. If it does, look for cracks in the hoses. Maybe spritz the suspects with brake cleaner, and you might see a temporary change if you hit the right spot.

    Not a Prius, but here's the general idea (he uses a smoke machine, but also talks a bit about spraying with brake cleaner.)

     
    #5 Danno5060, Oct 8, 2024
    Last edited: Oct 8, 2024
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  6. mr_guy_mann

    mr_guy_mann Senior Member

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    What brand Air Fuel sensor did you get & where did you buy it from? I recommend only using Toyota or Denso from an actual dealer or reputable online source (not amazon or ebay).

    You can use the search feature to look for posts I did in the Gen2 forums with "MAF readings". It's a chart with MAF sensor outputs at different RPM and load conditions. You can compare your readings at the same conditions to see how accurate your MAF is.

    As mentioned, getting Short and Long Term Fuel Trim numbers under different conditions can help direct the diagnosis. I like hot idle, 2000 RPM idle, moderate accel, and cruise (around 50-60mph).

    For example, restricted fuel injectors can cause high fuel trims under all conditions, while vacuum leaks mostly affect idle only.

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    #6 mr_guy_mann, Oct 8, 2024
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  7. Prius_let_me_go

    Prius_let_me_go Junior Member

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    Another update, I used some brand I found on RockAuto for the 02 sensor but it seems to be reading fine. The P0171 code went away and stayed off after disconnecting the battery the second time, but the P0420 is back, so it seems like the MAF cleaning worked, I am guessing it is just the cat now. But I will still check the A/F ratio sensor and the fuel trims.
     
    #7 Prius_let_me_go, Oct 15, 2024
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