P0420 Catalyst System Bellow Threshold Bank 1?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by KitsuneVoss, Jan 25, 2025 at 9:18 PM.

  1. KitsuneVoss

    KitsuneVoss Member

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    The code P0420 Catalyst System Bellow Threshold Bank 1, is that usually the upstream oxygen sensor?
    I know ti was with my 2001 Mustang.
    Also, are there any less expensive oxygen sensors than the Denso that work reasonably well?
    Looks I will have to do it myself because most mechanic shops now will not yet you buy the parts
    Make various bullshit claims but they will buy the part from an auto part store and triple the cost. .
     
    #1 KitsuneVoss, Jan 25, 2025 at 9:18 PM
  2. ChapmanF

    ChapmanF Senior Member

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    The ECM uses both the upstream and downstream sensors, and control of the engine mixture, to test the effectiveness of the catalytic converter, and report this code if that effectiveness has dropped below threshold.

    So the code is usually about the catalytic converter—that's the purpose of the test and the purpose of the code. Cases where the problem turns out to be one of the sensors just used for the test aren't impossible, but jumping there as a first guess would be like shooting the messenger. There are individual trouble codes for both sensors that are likely to be set if those aren't shipshape.
     
    #2 ChapmanF, Jan 25, 2025 at 9:39 PM
  3. mr_guy_mann

    mr_guy_mann Senior Member

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    The catalyst monitor test is fairly complex and requires a number of functional systems in order to be accurate.

    Replacing parts based on assumptions can: get expensive, not fix the problem, and cause new problems. (you might get lucky, but it's your money)

    When I diagnose a P0420, I make sure - 1) there are NO exhaust leaks anywhere at or upstream of the converter, 2) both A/F (S1 - upstream) and O2 (S2 - downstream) sensors can accurately report full lean to rich range, 3) the engine runs properly (total fuel trim is 0 +/- 10%, and NO misfires) 4) there are no applicable service bulletins (TSBs)

    IF all those are true then the car needs a new cat.

    5) if the engine uses-burns lots of oil - that killed the old cat and will kill a new one as well.

    Posted via the PriusChat mobile app.
     
    #3 mr_guy_mann, Jan 25, 2025 at 10:18 PM
  4. KitsuneVoss

    KitsuneVoss Member

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    Those answers may actually help me. I think the car is running a little on the rich side due to what appears to be low, but not crazy low, MPG compared to my previous car. What tends to cause a Prius to run rich?

    Did read MAF sensor and I plan to clean the MAF sensor soon. I think it is suppose to be 60 degrees Wednesday, so not a bad day to try to do it. Do I need to strip the throttle body and clean it? The same Mustang that I changed out the oxygen sensors, which were only like 20-30 each, I stripped the throttle body and cleaned it.

    I don't mind throwing a few dollars worth of parts at a car but the Denso oxygen sensors are really expensive but new gaskets for throttle bodies are generally a different category.
     
    #4 KitsuneVoss, Jan 25, 2025 at 11:03 PM
  5. MAX2

    MAX2 Member

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    #5 MAX2, Jan 25, 2025 at 11:22 PM
  6. douglasjre

    douglasjre Senior Member

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    You're wrong about everything you said for reasons you don't comprehend.
     
    #6 douglasjre, Jan 26, 2025 at 2:42 AM
  7. KitsuneVoss

    KitsuneVoss Member

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    Thanks, that is very unhelpful.
    Does it make you feel good just writing that?

    The thing is that my past experience with that code in a different vehicle was that it was the O2 sensor
    My other experience is that mechanic shops upcharge significantly the parts they buy, sometimes not just by 10% but by 100 to 200%.
    They used to also be more willing to put parts you buy for you.

    I write here to be helped, not to be called stupid.
     
    #7 KitsuneVoss, Jan 26, 2025 at 8:13 AM
    Last edited: Jan 26, 2025 at 8:27 AM
  8. frodoz737

    frodoz737 Top Wrench

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    Find a competent shop to T/S this before you just throw parts at it. That's my friendly.
     
    #8 frodoz737, Jan 26, 2025 at 10:17 AM
  9. KitsuneVoss

    KitsuneVoss Member

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    In the past, both on other cars and specifically with a Prius, I have found it cheaper to throw parts than to get a diagnostic check.
    The sensor is real expensive and the conversation here is that it is not likely to be the sensor.
    If there are a number of small things I can do that are not real expensive, I would like to try them first.
    Those are the answers I was hoping to get, little things I can try first.
     
    #9 KitsuneVoss, Jan 26, 2025 at 10:31 AM
  10. frodoz737

    frodoz737 Top Wrench

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    The system is actually very simple, but you need test equipment to T/S. Best wishes...I'm off.
     
    #10 frodoz737, Jan 26, 2025 at 10:36 AM
  11. KitsuneVoss

    KitsuneVoss Member

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    I have T/S actually. I thank Max2 for providing that sheet but I will have trouble following it.
    Is there a video which shows actually how to do it?
     
    #11 KitsuneVoss, Jan 26, 2025 at 11:56 AM
  12. rjparker

    rjparker Tu Humilde Sirviente

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    1. Does your engine burn oil?

    2. Have you checked the values produced by the monitor?
    IMG_7296.jpeg

    3. The aftermarket sensors can be a problem with bad quality. However I have used a Bosch aftermarket O2 on a gen2 after the cat and downstream sensor was stolen. The Bosch has different wire colors so read and follow the instructions. However as indicated above, a bad sensor or wiring will usually have its own code.

    Gen2 O2 Sensor to Bosch.jpeg

    4. The odds are 95-98% your set of cats are bad. If it's oil clogging your mpg and maybe driving reliability is at risk. After spending (wasting?) money on sensors you can get a new set of cats. Assuming you are in a non-CARB state, an aftermarket gen2 set of cats that works is a Walker 16337 ($160 - $271).
     
    #12 rjparker, Jan 26, 2025 at 12:39 PM
    Last edited: Jan 26, 2025 at 12:59 PM
  13. KitsuneVoss

    KitsuneVoss Member

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    I did just change out the spark plugs and did not seem especially bad. Have had it for about three weeks now. How would I verify if it is burning oil?

    Need to plug in the T/C and see if I can find the test screen you are referring to.

    How hard is the cats to replace? Looks a lot smaller than the one on my Mustang at least. It cracked on my mustang and jad to get it welded.
     
    #13 KitsuneVoss, Jan 26, 2025 at 1:59 PM
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