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.

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  1. 80sguy

    80sguy Junior Member

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    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

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    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

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    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
  4. 80sguy

    80sguy Junior Member

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    This is me testing it. I posted this twice came back to work to see if I didn't answers and I had no post
     
    #4 80sguy, Jan 20, 2026 at 3:46 PM
  5. Dany Dan

    Dany Dan Junior Member

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    #5 Dany Dan, Jan 20, 2026 at 4:55 PM
  6. Trollbait

    Trollbait It's a D&D thing

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    When were the spark plugs last replaced? The misfires could be causing the cat code, with the cat being fine.

    What is the actual temp of the downstream oxygen sensor? If below 600F, it isn't reading the exhaust. A clogged cat could cause that, but so can the sensor being gunked up or bad.
     
    #6 Trollbait, Jan 20, 2026 at 5:11 PM
  7. Tideland Prius

    Tideland Prius Moderator of the North
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    Threads merged
     
    #7 Tideland Prius, Jan 22, 2026 at 11:27 AM
    Brian1954 likes this.
  8. AzusaPrius

    AzusaPrius Senior Member

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    These catalytic converters are designed to only last at max 10 years.

    Your mileage may vary, depending on the health of the engine overall.

    Oil burner is a sure way to clog and over heat a cat.

    Misfires will also eventually cause damage.

    Then you also might have a head gasket leak which is causing the misfires.

    The check engine light will flash for the multiple misfires not the P0420, that will stay solid for that code.

    The inside honeycomb design will not melt into a blob and clog the cat, what it does do is crack and break up and you will eventually hear what sounds like sand or gravel under the car.

    You can try a bottle of Cataclean, this works and can remove the P0420 code but your misfire codes is another issue that should be looked at and delt with.
     
    #8 AzusaPrius, Jan 22, 2026 at 12:09 PM
  9. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Dependent on the miles and oil consumption as well? I doubt they have an expiration date, lol.
     
    #9 Mendel Leisk, Jan 22, 2026 at 1:49 PM
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