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New exhaust and still p0420

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by japeda, Jan 1, 2023.

  1. japeda

    japeda New Member

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    Hi all,

    Our 2007 Prius was throwing a p0420 code, and has been burning oil for many miles.
    I followed advice here and bought a cheap replacement cat on ebay and put it on and a new downstream o2 sensor. I should have done the autosaver88. I bought a "funsport" cat on ebay and it is a terrible replacement - the new exhaust is probably 4-5mm wider than the old, so they include an adapter that is 2 mm wider on either end, so even with two clamps and a lot of muffler sealant the new cat is leaking where it attached to the exhaust. The p0420 code stopped for a while but is back on intermittently. The old cat was very plugged with carbon. I cant imagine this new one would be so plugged after 1,000 miles, but maybe I'm wrong.

    My question:
    Would an exhaust leak downstream of o2 sensor cause the p0420 code to reappear? I want to weld on the muffler but wouldn't want to do that if the replacement is already showing a p0420. I suppose taking the new cat off again and seeing it's state of carbon would be a good first step. Obviously fixing the oil burn would be best but I don't have the time or money to do this now, hoping to either sell and buy a new one soon or get dirty this summer with piston rings.
     
  2. JC91006

    JC91006 Senior Member

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    Your problem may just be the "cheap cat from Ebay". They don't always do what they're suppose to do.
     
  3. Tombukt2

    Tombukt2 Senior Member

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    It is possible but don't fret I have the same problem and I bought the cat from the big cat manufacturer that sells on eBay and they make a cat for the gen 2. And it looked and fit perfectly it's very similar looking at the brick to the original it's sealed up fine front and back it's welded in place by my regular shop and the code never went away in this setup The downstream O2 sensor looks like it's doing its job and the numbers look okay in tech. But other than that the '0420 was always on and always standing still getting 47.1 miles to the gallon and I don't have emissions in Orange county North Carolina so I'm golden All I can think to do is take the check engine light bulb out or LED out I checked the car so often with code readers and tech that I really don't need the light the triangle pretty much do the trick. It must be the test that the system runs on the cat at each startup to me that's a farce somebody should be able to break the software code and figure out how to make that not happen or not need to happen some firmware update for something but that looks to be the issue.
     
  4. ChapmanF

    ChapmanF Senior Member

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    The test doesn't appear to run at startup. It will run at most once in a drive cycle, but only when the coolant is warmed above 75 ℃, the engine is running in closed loop, it's not idling and also not above 4000 rpm, the upstream and downstream sensors are both known to work, and both cat converters are in an operating temperature range.

    I took those conditions from a Gen 3 manual, but I doubt much is different there from Gen 2.

    And yes, "that looks to be the issue", in that if the car never tested cat function, you'd never see a P0420—same way halting COVID testing would have solved that problem.
     
    Tombukt2 likes this.
  5. Tombukt2

    Tombukt2 Senior Member

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    I don't think my Corolla and Camry and non-hybrid models check the catalytic converter especially as described for this vehicle before. So I don't know I just know the cat that I bought for my Gen 2 was from a pretty big manufacturer of catalytic converters throughout the US supply chain for lots of places so I would think they would have a clue what's happening and all of that maybe not. Oh well I don't have emissions everything is spot on otherwise. So I guess just keep getting it I can justify over $1,000 cat. But I can keep driving
     
  6. SFO

    SFO Senior Member

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    How many miles on the vehicle, and the old catalytic?
    Which o2 sensor did you purchase as a replacement, was it a Denso/OEM, and did you get it from the dealership or online?

    Why the downstream sensor and not the upstream, was it faulty or out of spec?
    Was that the originial OEM cat, and did you save it? (it has quite a bit of value as is, and you can also attempt to clean it for reuse)
     
  7. japeda

    japeda New Member

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    I bought a Denso o2 sensor from rockauto and replaced it. The downstream was also recommended to be replaced by users here just precautionary, I'm not sure if it was faulty. I took the OEM cat to a scrap yard for cash after replacing it.

    my options seem to be weld the aftermarket cat as is, or buy a new one from autosaver88 where people have claimed more success and weld that on or try with the adapter. Given the oil burn I think we'll just try to get through another year or so with this before moving on, and it would be great to not have a check engine light on when selling.
     
  8. SFO

    SFO Senior Member

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    You may need to use techstream (dealership technician diagnostic software) to test and monitor the vehicle. There are other options as well.

    Regarding the oil burn, have you thought about piston soaks, and or running special oil(s) to breakdown the carbon buildup?
    Upstream A/F sensors doesn't last forever, and some may show signs of failure sooner than downstream oxygen sensors.

    How many miles on the vehicle, and or the upstream A/F sensor?
     
  9. japeda

    japeda New Member

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    The vehicle has 212,500 miles and as far as I know it's the original upstream a/f sensor. The old cat was totally plugged with carbon from engine and was having serious power issues, which is what prompted the p0420 code and my replacement. I'd be interested in doing anything to get the engine running cleaner without having to spend too much money on it, but I've heard bad things about running special oils that break down gunk and end up clogging parts of the engine. I'd be interested to do compression tests to see what that turned up or try to diagnose a bad a/f sensor.

    [Regarding the oil burn, have you thought about piston soaks, and or running special oil(s) to breakdown the carbon buildup?

    Upstream A/F sensors doesn't last forever, and some may show signs of failure sooner than downstream oxygen sensors.

    How many miles on the vehicle, and or the upstream A/F sensor?[/QUOTE]

    It's possible the upstream a/f sensor isn't working well and could be the cause of the current p0420 code.