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Cat replacement (magnaflow) check engine light

Discussion in 'Gen 2 Prius Technical Discussion' started by davemo, Jun 22, 2021.

  1. davemo

    davemo Junior Member

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    Curious if any other gen2 owner who has replace the catalytic converter with a Magnaflow bolt-on OEM equivalent (as they describe it) have had the dreaded check engine light problem? My cat was stolen. Insurance offered a welded NAPA replacement that was a guaranteed check engine warning from reviews. OEM part was maybe early to mid July (again maybe). Looking at a lot of discussion here, I got the Magnaflow direct fit cat based on the reviews and recommendations by other Prius owners (avoided the dealer service markup by buying it myself). Already getting the check engine light error (code P0171 System too Lean) for oxygen sensor typical of aftermarket cats. I have run E15 the last few tanks too rather than my usual E10. ICE runs longer. Already experienced MPG drop prior to all this (more than E10->E15 would make energy hit), but it continues to run longer and MPG is dropping. Could be unrelated.

    Oh, and traction battery failed and was replaced a little over a year ago. 12v battery was a few years ago (memory has 3-4 years ago, but I have to go through my records), but I know this could be causing the ICE running longer & the resulting MPG dip. More interested in the warning light and not the likely unrelated ICE increase & MPG drop.
     
    #1 davemo, Jun 22, 2021
  2. Georgina Rudkus

    Georgina Rudkus Senior Member

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    Did you have a CEL before the OEM cat was stolen?

    The Gen 3 Prius might not be tuned for E15. What brand of oxygen sensor are you using? If it is a cheap Chinese one, it might be not up to OEM standards or may be defective.

    Other cheaper made Canadian and Chinese cats have functioned perfectly and not throw a CEL.
     
    #2 Georgina Rudkus, Jun 22, 2021
  3. davemo

    davemo Junior Member

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    O2 sensor was supposed to be a Denso or OEM (service writer & I discussed this prior to repair, and he actually was the one who advised me to order the cat myself to avoid his having to mark it up, but I'll have to double check this. The original sensor was ripped out during the theft. I have had a similar warning in the past, but again, that sensor is no longer among us due to the theft. I'll ask the service writer about what he used, although I'm guessing he'll tell me that it is likely the aftermarket cat, as he talked about how frequently this issue is a problem with non-OEM cats. Trying to sort out who is jerking your chain between service department, manufacturer of the part, and insurance company is shite.
     
    #3 davemo, Jun 22, 2021
  4. Georgina Rudkus

    Georgina Rudkus Senior Member

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    You could have an upstream failing oxygen sensor, since the ECU compares the upstream sensor to the output sensor.

    The only way to know is with Techstream ot an advanced OBDII diagnostic tool that can compare the readings.
     
    #4 Georgina Rudkus, Jun 22, 2021
  5. mr_guy_mann

    mr_guy_mann Senior Member

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    The O2 sensor that gets cut out with the cat theft is the downstream oxygen sensor. Primarily it is used to test "function" of the catalytic converter and really can't cause a P0171. IF there was a sizeable exhaust leak at the cat to exhaust manifold connection, that might contribute to the code- but you would easily hear the leak.

    Otherwise the cat theft and replacement should be unrelated to your code, you will have to diagnose it using the factory service manual.

    Posted via the PriusChat mobile app.
     
    #5 mr_guy_mann, Jun 22, 2021
    Georgina Rudkus likes this.
  6. Georgina Rudkus

    Georgina Rudkus Senior Member

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    My thoughts, too.
     
    #6 Georgina Rudkus, Jun 22, 2021
  7. davemo

    davemo Junior Member

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    Thanks for the info of the upstream O2 sensor. For those that know, is there a difference in the code that I should be alerted to when trying to see if this is a sub-standard cat vs the upstream issue?
     
    #7 davemo, Jun 24, 2021
  8. mr_guy_mann

    mr_guy_mann Senior Member

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    Well, usually the most common fault of a substandard cat is that it will trigger a P0420 code (low catalyst efficiency).

    A P0171 lean run code can be set by many different things: bad MAF or upstream A/F sensor, vacuum leak, fuel injectors, fuel pump, bad gas, etc. Read the service manual. All of these things affect engine operation BEFORE the exhaust gases reach the cat. So the catalyst operation has no affect on upstream fuel control (fuel trim) and can't cause a P0171.

    Posted via the PriusChat mobile app.
     
    #8 mr_guy_mann, Jun 25, 2021
    Last edited: Jun 25, 2021
  9. Prius RT

    Prius RT Junior Member

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    Your p0171 got resolved ?
    I have a similar story. Replaced CAT with Walker and also downstream O2 sensor Denso. Now I get p0171. No major vacuum leak except some through the valve cover gasket which was replaced 6 months ago. Mpg has dropped from 43mpg to 36mpg.
     
    #9 Prius RT, Aug 9, 2023
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