P0303 Code

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by veggiecar, Feb 16, 2026.

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  1. ASRDogman

    ASRDogman Senior Member

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    That's good. But you don't know what the issue was....
     
    #21 ASRDogman, Feb 19, 2026
  2. rjparker

    rjparker Tu Humilde Sirviente

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    It was the #3 coil
     
    #22 rjparker, Feb 19, 2026
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  3. veggiecar

    veggiecar Junior Member

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    Here we are about five and a half months later. My son started the car last night and I heard the motor shutter and shake for a moment and then ran smoothly. When he got home the check engine light was on. The code was p0300, p0302, p0303, p0304. I cleared the code and restarted the car and without any misfires the same codes reappeared. Five months ago I replaced the spark plugs and coils from amazon. Marsram Ignition Coil UF596 Spark Plug. Amazon said they would fit my 2012 Prius. Is it possible that these parts are substandard or is there another potential explanation. Thanks so much for the help…
     
    #23 veggiecar, Aug 3, 2026
  4. ASRDogman

    ASRDogman Senior Member

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    It is VERY likely you got cheap chinese copies. Surprised they lasted that long.
    Do you still have the OEM coils? If so, put them back in.
    Go to autozone and get the Bosch Double Iridium spark plugs.

    NEVER buy electrical parts from ebay or amazon. 99% of them are cheap chinese copies.

    And you still don't know what the cause of the problem was/is.

     
    #24 ASRDogman, Aug 3, 2026
  5. veggiecar

    veggiecar Junior Member

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    I could put the plugs back in but one of the coils is definitely bad. I will get a name brand product.
     
    #25 veggiecar, Aug 3, 2026
  6. ASRDogman

    ASRDogman Senior Member

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    There is no sense in getting a cheap ones.
    Unless you like taking the wiper assembly apart....

    I know it cost more to get the parts from Toyota, but at least you know they are good parts.
    And you won't have to change them again for 100,000 miles. And maybe never for the coils.

     
    #26 ASRDogman, Aug 4, 2026
    Last edited: Aug 4, 2026
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  7. BiomedO1

    BiomedO1 Senior Member

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    The morning shutter and shaking doesn't bode well. Did you test for head gasket failure and did you keep the old parts? I'd stick with the OEM stuff - proven track record. I toss them when I know they are bad and use them for troubleshooting; when I suspect an issue. This is better than throwing Amazon or Ebay parts at a problem...... IMHO
     
    #27 BiomedO1, Aug 4, 2026
  8. veggiecar

    veggiecar Junior Member

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    1. I went to autozone and bought the most expensive coils that they had. These coils are made for autozone and have a lifetime warranty. Unfortunately these new coils did not fix the problem. The code was P0304. I swapped out the number 4 cylinder coil with an OEM that was working back in February. This did not fix the problem. I changed the spark plug and that too did not make the problem completely go away. The problem is not ongoing. It only occurs when the car begins to run and only for 1 to 3 seconds and then it runs pretty good. Are there any other tests that I could do to narrow down the problem? How do you do the leak test that I have heard mentioned?
     
    #28 veggiecar, Aug 14, 2026 at 6:05 PM
  9. rjparker

    rjparker Tu Humilde Sirviente

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    Most likely you have a very small head gasket leak but now it has progressed.

    If it is losing ANY coolant that is a pretty solid indication. If it fails a chemical combustion gas in the coolant test it is another firm indication. However if you are not losing coolant or it passes the combustion gas test, you can still have a head gasket leak.

    The definitive test is a high quality borescope inspection performed on a cold engine and a manually pressurized coolant system.

    Two videos:

    Car Care Nut HG Borescope at 660s


    South Main Auto
    Excellent Borescope Footage at 6:50
    Honda Misfires on Coldstart Only


    There are two borescope indicators but for best results, a high resolution dual view or articulating borescope is recommended.

    One indicator is a "steam cleaned" piston where some or all of the piston's aluminum surface has been washed off of its normal carbon buildup.

    Another indicator is actually seeing coolant on the piston or drips from the head gasket. This requires a decent borescope.

    Finally doing nothing will often lead to a worse situation which can include severe overheating, warped heads, bent rods or a hole in the block. All of these will require a replacement engine.

    In some cases a replacement JDM engine is a better solution than a quality pro head gasket job since it will normally have no oil burning or leaks.

    “Hey Walt… they want to wait and see if it gets worse…” (and it will with examples)
     
    #29 rjparker, Aug 15, 2026 at 1:10 AM
    Last edited: Aug 15, 2026 at 1:19 AM
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