Rear Cat - not plugged but honeycomb rattling around

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by NixieOP, Feb 6, 2026 at 1:18 PM.

  1. NixieOP

    NixieOP New Member

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    There are numerous cat threads but no one seems to talk much about the rear cat. My Prius has a rattle at certain rpms that I traced to the rear cat. The catalytic converters have clearly been replaced with off-brand; I know from my experience with a Subaru that the honeycomb in low quality cats will sometimes separate from the housing and rattle around. No engine codes, I'm assuming because there's no sensors there.
    My questions are: since the second cat is after any sensors, is there any way to tell if it's plugged or not functioning well? And would it be dangerous to the systems to drive as-is?
    I don't know for sure that it's broken, but the rattle from inside seems telling. Do I just live with the noise?
    Thanks for your time.
     
    #1 NixieOP, Feb 6, 2026 at 1:18 PM
  2. JC91006

    JC91006 Senior Member

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    You probably have an oxygen sensor extender on the cat, so that it cannot detect the emissions and thus you have no check engine light. The ratting is probably from the extender, which often has a cap, that came off. That cap is breaking apart the honeycomb and if it doesn't eject itself, it will break up the honeycomb enough to clog the catalytic converter completely. Once clogged, your engine will die. I've had this happen so I know all the details.
     
    #2 JC91006, Feb 7, 2026 at 11:47 AM
  3. NixieOP

    NixieOP New Member

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    Yikes. What do I do?
     
    #3 NixieOP, Feb 7, 2026 at 12:13 PM
  4. BiomedO1

    BiomedO1 Senior Member

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    Just pull it down and empty it out. Simple.... That way it doesn't rattle or clog-up you exhaust passage.
    Now if they do smog inspections in your state, that's another matter....
     
    #4 BiomedO1, Feb 7, 2026 at 2:22 PM
  5. JC91006

    JC91006 Senior Member

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    [​IMG]
    My guess is that cap in the picture, is what came off and rattling. The solution would be to remove the entire cat and hopefully shake that cap (or whatever is loose) out of there.
     
    #5 JC91006, Feb 7, 2026 at 4:31 PM
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