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New Member with a P0420 query

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Tynyyn, Jul 2, 2018.

  1. Tynyyn

    Tynyyn Member

    Joined:
    Jul 2, 2018
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    Location:
    Bella Vista Arkansas
    Vehicle:
    2004 Prius
    Model:
    II
    Good day ladies and gentlemen.

    I've recently purchased a '04 Prius base with 239K on the clock. I am probably the third owner of this vehicle and the PO thrashed the interior to my dismay. But the gentleman I purchased it from said he installed a replacement HV battery and that was good enough for me to overlook the stains on the seats and the brown "stuff" in the carpets.

    I've only put about two hundred miles on this car and the P0420 code popped up. Yes, I have read several threads on PriusChat about this code and the various ways of fixing the problem. I only have a Harbor Freight scan gauge and it only tells what code has arisen. The fancy dan super dooper scan gauges which show voltages and which banks the problem is occuring on is not on my wish list for Christmas because I know there are folks a lot smarter than I who have the knowledge in their heads and generally are willing to pass it on.

    So here is my question:.. A lot of the senior members will say that the cat is dead when a P0420 pops up and they hardly go into their reasoning behind this assertation. I certainly understand they have the "been there, done that", but how do they KNOW that the upstream sensor isn't dead or dying? Do the sensors never die? Are they more hardy than the cats? Just a curious observation I've contemplated for a couple of days, now.

    I've been on other automotive forums where the older members school the newbies and say "if you are having ABC problem, then the cause is XYZ. And there statements turned out to be true. But with the cats on the Gen2 prius, why do the older members always seem to go straight to the dead cat instead of having the newbie somehow test the O2 sensor or just say "replace the O2 sensor"? What knowledge do the older members have that they absolutely know the cat is dead?

    I've narrowed down the P0420 code to four possible causes:
    1. Oil consumption and it deteriorated the cat
    2. Air leak causing a lean burn
    3. Dead upstream sensor
    4. Dead cat(probably due to oil contamination + lots of miles)

    I've had the car for such a short amount of time I cannot say if it is burning oil, that will take a couple of weeks. I tightened up all of the air intake gaskets and cleaned the MAF sensor. I don't know if the O2 sensor is wonky since I ain't got a fancy scan gauge. And the cat is now the most prevalent culprit on the short list.

    Comments? Suggestions? Recommendations?

    And if you are a senior member, please do not be offended by my choice of phrases. I'm only trying to get a certain point across that you guys are wonderful fonts of information and I'd like to know how you came up with your conclusions.

    Have a great day.
    Terry
     
  2. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
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    Location:
    Los Angeles, CA
    Vehicle:
    2008 Prius
    Model:
    II
    Terry

    With the high mileage, chances are you have a bad cat. This will not disable your car, so just continue driving it and monitor the oil level. Then you'll know what you have on your hands before you start putting money into the repairs.

    No purpose to repairing a catalytic converter when the engine burns tons of oil, right? Maybe you would need to do the engine repair before the catalytic converter (if it's even that). So I recommend just driving it for a while.......
     
    Raytheeagle likes this.
  3. SFO

    SFO Senior Member

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    Vehicle:
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  4. Patrick Wong

    Patrick Wong DIY Enthusiast

    Joined:
    Mar 8, 2008
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    Location:
    Green Valley, AZ
    Vehicle:
    2015 Prius
    Model:
    Two
    An air/fuel ratio sensor is not very expensive so go ahead and replace it first, if you like. It won't hurt and might help.

    If you wanted to test the sensor, you would need a means of reading the voltage produced by the sensor while the engine is operating, which implies you have access to Toyota Techstream (Mini VCI is a clone) or similar software.
     
    SFO and ITBland like this.