My second head gasket (relatives car) didn't go well

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by C Clay, Apr 12, 2026 at 3:00 PM.

  1. C Clay

    C Clay Member

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    I helped a relative do his head gasket on a 2010 two weeks after I did mine. Everything went well except for the timing was off. So I cracked it back open and did it right. I admit to not watching closely when I put the crank pulley back on and turning it two revolutions. I was apparently off. The second time was the charm.

    Except for a flashing check engine light and the following codes-p304 and p0420. The car sat then a couple of weeks later I was able to take a look. We swapped coils, even swapping spark plugs, and even swapped a couple of fuel injectors (a suggestion I read somewhere).

    Never have had the check engine light come on again, and at some point the p304 code went away- but never the 420. It's running a little rough at say 30 mph plus and a we are getting misfires (as reported via Car Scanner app) on cylinders 2,3,4. At one point it was 4 only.

    I did not now until we were finished- the driver had already been told "you need a catalytic converter". I never drove it before we did the head gasket and I never knew what codes it had before we did the head gasket. It certainly did need the job done-that was advised by the shop, but the customer declined.

    I love a little help from AI- based on what we've done so far, she is suggesting:
    Summary of Next Steps:

    1. Prioritize the Exhaust: Crack the exhaust flange open and test drive (or at least rev the engine) to see if the sluggishness disappears.
    2. Verify the Cat: If the P420 was there before the head gasket failure, the coolant that leaked probably finished off an already aging converter.

    Would the fine scholars and engineers here at Priuschat recommend the same steps or some other course of action?


    Some of you might remember I did a successful head gasket replacement on my 2014, totaled the car 3 days later... I just purchased this week my Prius 2.0- a 2019 that is an a very stripped down fleet model, but has all the goodies I really wanted- adaptive cruise, collision avoidance, blind spot, auto dimming headlights, and the parallel parking thing!
     
    #1 C Clay, Apr 12, 2026 at 3:00 PM
  2. BiomedO1

    BiomedO1 Senior Member

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    I don't understand the reasoning behind focusing on the CAT when you've got misfires to take care of.?????
    Those misfires (rich gas mixtures) will simply FRY a new CAT anyways. Or if you need to replace the engine because you didn't check for a warped head or block - are you willing to spend all that extra money too????

    People needs to realize that if you don't ask AI a question correctly; your not going to get the correct answer. Garbage in Garbage out....

    YMMV
     
    #2 BiomedO1, Apr 12, 2026 at 3:29 PM
  3. C Clay

    C Clay Member

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    Help me diagnose the misfires. I can probably investigate how old the spark plugs are, the coils look new. I’m told both are fairly new.
     
    #3 C Clay, Apr 12, 2026 at 3:32 PM
  4. BiomedO1

    BiomedO1 Senior Member

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    You need to test them; don't care how good they look. You can put all the lip-stick on a pig you want - It still isn't going to taste like beef....
    You sure you didn't bend any valves when it was off-time???? Those are interference engines.
     
    #4 BiomedO1, Apr 12, 2026 at 3:37 PM
  5. PriusCamper

    PriusCamper Senior Member

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    This misfire code went away, correct? If so finish the job by putting a cheap aftermarket cat replacement on the car... I'm not really sure what you're asking? The failed cat was a known problem before the headgasket, why ask for help when you already know the fix?
     
    #5 PriusCamper, Apr 12, 2026 at 4:19 PM
  6. Tombukt2

    Tombukt2 Senior Member

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    It was time to get out of that gen 3 at the first sign of your mistake or whatever. Now you're just gonna be behind if it ever get fixed .
     
    #6 Tombukt2, Apr 12, 2026 at 5:01 PM
    BiomedO1 likes this.
  7. BiomedO1

    BiomedO1 Senior Member

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    Just because the CEL isn't lit; doesn't mean there isn't a problem. It just means that it hasn't error-ed enough to set a code yet. Bet there's a bunch of pending codes.
    If the CAT was obstructed/collapsed, there would be a power issue and the scanner wouldn't report misfires. You can disconnect the exhaust pipe to remove the back pressure to test. If the misfires are still there; that has to be addressed FIRST.... Otherwise, as I stated earlier; your just going to fry that new CAT.
     
    #7 BiomedO1, Apr 12, 2026 at 5:36 PM
  8. rjparker

    rjparker Tu Humilde Sirviente

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    It helps to know the basics like oil burning and mileage.

    My guess is the 2010 burns a lot of oil, clogged up the cat and everything else including the intake and valves. I will assume its over 200k miles.

    It probably ran low on coolant and overheated.

    After the hg job twice over it still misfires but not enough for a check engine. Which sounds like the hg job failed perhaps due to a warped head.

    I would replace the engine and install an aftermarket cat. I would not do the cat first hoping that would fix the misfires.

    Or junk it.

    IMG_1130.jpeg
    This brand has kept the light off and claims to be epa compliant
     
    #8 rjparker, Apr 12, 2026 at 8:48 PM
  9. WICruiser

    WICruiser New Member

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    I realize that you may not have all the information but to anwer your question we really need to know more about the car; miles, oil consumption, coolant loss, history of codes, etc.

    Also clarify but as I understand the situation this car has had one head gasket replaced (some seem to believe it has had 2?) and after the job was complete and the timing corrected that codes remain. If the codes are carried over from before it means that you did not disconnect the 12v battery, so if you disconnected the battery the codes are new.

    Just because someone said the convertor needs to be replaced doesn't mean it needs replacement but as AI suggested if the failed head gasket resulted in a fair amount of coolant entering the exhaust it could kill the convertor. However, if the recommendation to replace the convertor was part of the original head gasket diagnostics I would suspect it was a WAG and not follow that without more evaluation.
     
    #9 WICruiser, Apr 13, 2026 at 8:00 AM
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