Consistent P0420, sudden misfires,P0301+P0303,fixed,now still misfiring

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by catteridge, Apr 3, 2025.

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  1. catteridge

    catteridge Junior Member

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    Hello all,

    I have an 08 with 276k miles.
    I've been driving with P0420 coming on and off for about 9 months now.
    Very tight budget and have been living in CA so haven't replaced the cat.
    I would typically reset it when it doesn't go away from a while.

    I was having issues with my Torque Pro app and was on a cross country drive with the CEL on, and couldn't reset the code of check for new codes.
    I was almost to my destination last week when slight shuddering began that I actually thought was the road I was on at first...
    then within 30mins it pretty quickly increased in intensity so I looked up the symptom quickly and saw that if the CEL is blinking it indicates an immediate serious problem.
    I look up and the CEL has begun blinking and then the array of ABS and other dash lights comes on.
    I pull over for a while then drive it to nearby town with cell signal (shuddering was less sever now and dash lights had gone away, except solid CEL).

    Auto shop read codes 0301, 0303, and p420.
    Replaced all spark plugs and coils 1 + 3, cleaned MAF.

    Drove another 2 hours when small shuddering started and kept increasing. I stopped and someone I know came and towed my car the remaining 4 hours to my destination.

    Now I can drive it around, only code is p420, but there is the slightest hint of shuddering still.


    Now I am wondering if replacing the catalytic converter at this point will solve the misfiring or if there is other stuff happening, possibly as a result of me not addressing the p0420 for such a long time.

    I am aware misfiring can cause damage to the cat, but could ignoring a bad cat cause the misfires?

    Probably looking at selling this car at this point.
     
    #1 catteridge, Apr 3, 2025
  2. Tombukt2

    Tombukt2 Senior Member

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    Next thing I would do is be pulling the plugs in the morning and looking for any kind of moisture or anything I can see fine droplets on the plug tips shuttering generally has to do with imbalance and all that kind of funny business either by ignition the fire being put out by moisture things of this nature at 300k prime candidate for this kind of stuff really so you want to rule the moisture out whether you have an inspection camera or you pull the plugs in the morning after a good drive in the evening that'll let the condensation usually set up pretty good give you an idea of what you might be up against It's really uncommon for this engine generally speaking but when you start getting 300 plus thousand things can happen just because it's nothing personal or anything like that it's just possible I've got an '09 car right now that I believe is having very similar issues and I'm thinking it's probably head gasket also
     
    #2 Tombukt2, Apr 3, 2025
  3. catteridge

    catteridge Junior Member

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    Thanks for the response. What would moisture on the plug tips indicate as the problem and accompanying solution?
     
    #3 catteridge, Apr 3, 2025
  4. MAX2

    MAX2 Senior Member

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    In a working car, everything works properly.

    The exhaust gases from the engine will be within the normal range and the catalytic converter will clean them.

    When problems start in the engine, the exhaust gases get worse, the catalytic converter can no longer cope with the task and reports the error P0420.

    This is the first bell that says that it is necessary to thoroughly check the engine system and the fuel-air mixture.

    You have already had misfires, you have carried out preventive measures, but they may be insufficient, and the engine errors will return again.

    I recommend checking all systems according to the manual for ignition propulsions.
     

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    #4 MAX2, Apr 3, 2025
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