2010 Prius – Misfire (Cyl 1 & 3) / P0403 – Head Gasket or EGR?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Hdjdhdh, Apr 16, 2026 at 10:41 PM.

  1. Hdjdhdh

    Hdjdhdh Junior Member

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    Location:
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    Vehicle:
    2010 Prius
    Model:
    IV
    Trying to figure out if this is a head gasket or EGR issue.
    Symptoms:

    • Misfire codes on cylinders 1 and 3

    • P0403 code (EGR circuit malfunction)

    • White smoke on startup that goes away after a while

    • No rough idle and no rough acceleration (mostly drives smooth otherwise)

    • Shudder / shake and slight knock at certain throttle positions

    • Coolant loss about 6 months ago, but level has not dropped since refill

    • No milky oil
    Testing so far:

    • Leak-down test came back normal

    • Unplugging the EGR valve: shudder/misfire under load goes away when disconnected

    • Engine runs noticeably better with EGR unplugged
    Question:
    Does this point more toward a clogged EGR system / intake issue, or could this still be an early head gasket issue even with a normal leak-down test?
     
    #1 Hdjdhdh, Apr 16, 2026 at 10:41 PM
  2. Brian1954

    Brian1954 Senior Member

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    How many miles or km on the car?
     
    #2 Brian1954, Apr 17, 2026 at 8:08 AM
    Mendel Leisk likes this.
  3. Mendel Leisk

    Mendel Leisk MMX GEN III

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    EGR, including intake manifold, ever been cleaned?
     
    #3 Mendel Leisk, Apr 17, 2026 at 8:26 AM
  4. WICruiser

    WICruiser New Member

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    Unless the EGR cooler has failed and allows coolant to enter the intake system there is no explanation for the coolant loss other than a head gasket. Your white smoke on start up is an indication of coolant in the cylinders, most likely from a head gasket failure, but given that you don't mention having rough idle on start up it could indicate the problem is in the EGR cooler and it takes time to get to the combustion chambers.

    Probably worth removing, cleaning, and testing or just replacing the EGR cooler (follow Mendel Leisk's advice on removval, etc.) to see if that is the problem before investing in a head gasket replacement.
     
    #4 WICruiser, Apr 17, 2026 at 8:52 AM
  5. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Probably head gasket has failed though, and for some time, considering multiple cylinder misfire codes.
     
    #5 Mendel Leisk, Apr 17, 2026 at 9:24 AM
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