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P0420 with new cat and sensor o2 sensor?

Discussion in 'Gen 2 Prius Technical Discussion' started by ChaseE, Apr 3, 2022.

  1. ChaseE

    ChaseE Junior Member

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    2005, check engine light tripped code P0420, research points to bad catalytic converter or o2 sensor...50/50 city/hwy MPG is 53 per tank. Thing is, I just replaced the catalytic converter and o2 sensor 2000 miles ago. Posted images from torque pro on the voltage readings for the upstream A/F sensor and o2 sensor...noticed while driving with ICE running at 55mph the O2 sensor voltage will occasionally bounce from 0.7 down to 0.1, so maybe bad electrical connection tripping the code? Also, isn't 4.0 volts for the upstream A/F sensor above nominal, as in a lean condition? A/F ratio cruising at 55 mph is around 14.65-14.7.
     

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    #1 ChaseE, Apr 3, 2022
  2. SFO

    SFO Senior Member

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    What brand and model# upstream sensor did you purchase, and where from?
     
    #2 SFO, Apr 3, 2022
  3. Another

    Another Senior Member

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    Did you DIY or have someone do it? If the latter, go back and ask them to diagnose.
     
    #3 Another, Apr 4, 2022
  4. ChaseE

    ChaseE Junior Member

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    It was a Denso model 234-4623, purchased from Amazon and it was direct from Amazon not a third party. Only replaced the downstream o2, not the upstream A/F sensor.

    This was DIY, super easy installation.
     
    #4 ChaseE, Apr 4, 2022
  5. Classic Car Guy

    Classic Car Guy Active Member

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    wow you got an excellent mpg which is great...
     
    #5 Classic Car Guy, Apr 4, 2022
    Last edited: Apr 4, 2022
  6. 2010moneypit?

    2010moneypit? Active Member

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    Some after market cats will trip the PO 420 code. They do not have sufficient media inside of them to clean up emissions from the ice.
     
    #6 2010moneypit?, Apr 4, 2022
  7. SFO

    SFO Senior Member

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    How many miles are on the vehicle?
     
    #7 SFO, Apr 4, 2022
  8. dolj

    dolj Senior Member

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    It's P-zero-four-two-zero—P0420.

    The second character is always a numeric, specifically either a 0, a 1, a 2, or a 3.

    Getting this right can make a big difference to what you can dig up when researching.
     
    #8 dolj, Apr 4, 2022
  9. 2010moneypit?

    2010moneypit? Active Member

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    0k.
     
    #9 2010moneypit?, Apr 4, 2022
  10. mr_guy_mann

    mr_guy_mann Senior Member

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    When you are driving with the ICE running at cruise or slight accel, the upstream A/F sensor (B1S1) should read about 3.3V. It will read higher when it reports a lean condition, up to 5.0V during fuel cut (ICE off).

    Posted via the PriusChat mobile app.
     
    #10 mr_guy_mann, Apr 5, 2022
  11. ChapmanF

    ChapmanF Senior Member

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    See what you did there.
     
    #11 ChapmanF, Apr 5, 2022
    2010moneypit? likes this.
  12. ChaseE

    ChaseE Junior Member

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    Updated to a very old thread...haven't seen P0420 since clearing codes a while last year but now see P0137 on occasion with lower MPG, average with 50/50 City highway mix is around 49 when it used to be 53...screenshot of torque pro readings during a 2-hour drive at 70 mph. AF sensor fluctuating between 3.9 to 4 volts, A/F ratio around 13.9-14, downstream 02 sensor showing either 0.1 volts or 0.8 volts...nothing in between. Thinking I might have to replace both the O2 and air fuel sensor but want opinions first since replacing the air fuel sensor will probably be a PITA
     

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    #12 ChaseE, May 16, 2023
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