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Is this the right way to diagnose P0420?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by jenesuispasbavard, Jul 25, 2022.

  1. jenesuispasbavard

    jenesuispasbavard Junior Member

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    Hello fellow Gen 2'ers, I've recently got the P0420 error code for "Catalyst System Efficiency Below Threshold (Bank 1)". RIP.

    After some reading on these forums, I'm trying to figure out whether it's the upstream A/F sensor or the downstream O2 sensor, or worst of all the catalytic converter itself that's at fault. But I'm getting some weird readings from the sensors that I wanted to run by y'all.

    Here are some data plotted vs time. The first image ("there") is about 15 minutes where I was occasionally going on and off the gas pedal on the highway to see what each sensor does. The second image ("back") is another 15 minutes with a more engaged gas pedal and loaded engine.
    Screen Shot 2022-07-25 at 15.52.35.png
    Screen Shot 2022-07-25 at 15.53.21.png

    The upstream sensor is at 3.95-4V at steady state. Shouldn't it be 3.3V? I'm plotting these PIDs with the Torque app:
    • [PRIUS]Engine Speed(RPM),
    • O2 Bank 1 Sensor 1 Wide Range Voltage(V),
    • O2 Bank 1 Sensor 2 Voltage(V)
    Any idea if this is one of the O2 sensors or the cat itself that has an issue?

    If someone wants to see more data, I also logged the following:
    • [PRIUS]Accelerator Pedal Angle(%),
    • [PRIUS]Combined Power (hp)(hp),
    • Air Fuel Ratio(Commanded):)1),
    • Air Fuel Ratio(Measured):)1),
    • Catalyst Temperature (Bank 1 Sensor 1)(°F),
    • Catalyst Temperature (Bank 1 Sensor 2)(°F),
    • Fuel Trim Bank 1 Long Term(%),
    • Fuel Trim Bank 1 Short Term(%),
    • Mass Air Flow Rate(g/s),
    • O2 Bank 1 Sensor 1 Wide Range Equivalence Ratio(λ),
     
    #1 jenesuispasbavard, Jul 25, 2022
    Last edited: Jul 25, 2022
  2. jenesuispasbavard

    jenesuispasbavard Junior Member

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  3. PriusCamper

    PriusCamper Senior Member

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    Everyone wants to blame the sensors when their cat goes bad... It's a waste of time and money thinking that. Especially when a quality replacement cat w/warranty like this one: https://www.ebay.com/itm/193690871378 only costs $130 and takes less than a couple hours to install. Heck, they even sell $60 replacement cats these days. Of course make sure the engine has new spark plugs and is running clean first.
     
  4. mr_guy_mann

    mr_guy_mann Senior Member

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    If short and long term fuel trims are mostly 0 +/- 10% then it looks like Torque doesn't report the A/F sensor data correctly. 4.0V would be way lean (around 3.3V is normal for 14.7:1) but the S2 O2 sensor looks kinda normal (700-ish mV) - except for a fair amount of switching.

    Does the measured ratio kinda match up with the commanded ratio? If so and there are no exhaust leaks near the cat or S2 O2, then the cat looks weak.

    Posted via the PriusChat mobile app.
     
    jenesuispasbavard likes this.
  5. jenesuispasbavard

    jenesuispasbavard Junior Member

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    I mean I'd rather spend $50 than $200 incl. welding if the problem is with a sensor rather than the cat - unlikely I know. And plotting graphs is fun (to me)!

    In the following pictures, "there" route was variable engine load, "back" route was more constant load.

    Looks like Long term fuel trim stays near 9.375% for most of the time, and both short- and long-term fuel trims stay mostly within ±10%.
    Screen Shot 2022-07-26 at 10.14.16.png
    Screen Shot 2022-07-26 at 10.23.39.png

    Measured ratio does match up with commanded ratio when engine is under load, but not when I let off the gas (not sure if that's normal).
    Screen Shot 2022-07-26 at 10.14.59.png
    Screen Shot 2022-07-26 at 10.15.07.png

    Interactive plots here:

    Guess it's the cat, RIP. :cry:
     
  6. bisco

    bisco cookie crumbler

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    at least you can sell it for big bucks and install a cheap aftermarket
     
    jenesuispasbavard likes this.
  7. PriusCamper

    PriusCamper Senior Member

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    The aftermarket replacement cats are direct fit and bolted to the resonator side of the pipe with a pipe clamp. And as Bisco mentions, the 2 grams of Rhodium, 2 grams of Platinum and 2 grams of palladium in Gen2 Prius cats makes them worth big bucks. More than a year ago people were selling them for $1500 cash. But due to all the thefts and new laws, you'll probably barely get $1K. But that's more than enough to cover your costs.
     
    jenesuispasbavard likes this.
  8. jenesuispasbavard

    jenesuispasbavard Junior Member

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    Good to know there's no welding involved! How long do these aftermarket cats last compared to OEM? Judging by the service history I think this is the original cat in this 203,500-mile Prius (I bought it at 142k). I don't expect the new cat to last another 200k.

    I guess I should wait a while to see if my engine is burning any oil too (so far I had a spell of 1qt/3000 miles but that seems to have decreased after a round of BG EPR/MOA/44K). State emissions inspection isn't due for another three months.
     
  9. Joao

    Joao Junior Member

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    Hello, I have similar data in torque pro and also p0420. Did replacing the cat solve this?