Rear Cat - not plugged but honeycomb rattling around

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by NixieOP, Feb 6, 2026.

  1. NixieOP

    NixieOP New Member

    Joined:
    Feb 6, 2026
    2
    0
    0
    Location:
    Washington State
    Vehicle:
    2004 Prius
    Model:
    N/A
    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
  2. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    16,805
    8,592
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2008 Prius
    Model:
    II
    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
  3. NixieOP

    NixieOP New Member

    Joined:
    Feb 6, 2026
    2
    0
    0
    Location:
    Washington State
    Vehicle:
    2004 Prius
    Model:
    N/A
    Yikes. What do I do?
     
    #3 NixieOP, Feb 7, 2026
  4. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    3,992
    2,100
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    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
  5. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    16,805
    8,592
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2008 Prius
    Model:
    II
    [​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
(You must log in or sign up to post here.)