P0420 + P0300 is it the chicken or the egg?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by 80sguy, Jan 20, 2026 at 9:40 AM.

  1. 80sguy

    80sguy Junior Member

    Joined:
    Jan 20, 2023
    30
    4
    0
    Location:
    Rome NY
    Vehicle:
    2008 Prius
    Model:
    Five
    My wife's 2011 Prius had been running fine. 221K and counting. My son was driving it in the engine light came on flashed and then went solid. I told him to pull over and I'd come meet him. I pulled the codes and there was the cat code p 0420 as well as random misfire and misfires on cylinder 1 2 and 3. (P0300,301,302,303)
    The scanner also pulled the temperatures and it was about 180° higher at the front of the cat then at the back . My understanding is that is a sign of a bad cat. I've heard but not sure I understand why that would make the engine misfire other than perhaps back pressure? I'm thinking is that the other way around that misfires leading to unburnt fuel possibly clogging the cat. I just don't know why it would happen all at once. What do I address first? I've had good luck in removing cats before and cleaning them. Thanks to all
     
    #1 80sguy, Jan 20, 2026 at 9:40 AM
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    27,753
    18,327
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Persistent misfiring over a sustained period does heat up the cat, because the fuel that wasn't burned in the misfires ends up in the cat and burns (or, ok, exothermically reacts...) there, making the cat very hot.

    The fuel itself doesn't clog the cat, but if the cat temperature gets hot enough to melt the internal honeycomb structure into a big blob, that clogs the cat.

    P0420 isn't really a code about clogging, but about the function of the cat being degraded.

    Something worth remembering about the OBD-displayed cat temperatures is that there really aren't any direct temperature sensors there. Those temperature readings are estimated/predicted/calculated by the ECM based on other readings it actually knows, like airflow and fuel/oxygen content of the exhaust.

    The check-engine light comes on solid for most engine codes. The only time the ECM chooses to flash the check-engine light is when it predicts/estimates/calculates a cat temperature so high that it risks damaging the cat, such as when the ECM observes sustained serious misfiring.
     
    #2 ChapmanF, Jan 20, 2026 at 10:55 AM
  3. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    3,817
    1,984
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    Don't forget about the oil burn issue too, that will also take-out a CAT.

    Why is the car misfiring???? Putting a band-aid on surface issues isn't going to fix the underlying cancer beneath. IMHO...
     
    #3 BiomedO1, Jan 20, 2026 at 12:55 PM
(You must log in or sign up to post here.)