P0420 Code only when there is less than half tank of fuel

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

  1. Koich

    Koich New Member

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    Hello all. This is my first post.
    I searched the forum to look for similar symptoms, but couldn't find any.

    Because the CEL/P0420 only comes on after consuming more than half the tank of fuel, I'm assuming it's something tank-related. I learned that the Gen 2 Prius doesn't have an EGR valve, so assuming it's something else but fuel-related.
    My previous experience with Toyotas with similar problems was fixed with a new gas cap. Maybe I'll try this for the first shot.

    Has anyone experienced anything like this?
     
  2. Brian1954

    Brian1954 Senior Member

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    Does the car have the original catalytic converter? How many miles on the car? Does your engine burn oil?
     
  3. frodoz737

    frodoz737 Top Wrench

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    ...and this CAT code goes away by itself when you fill up?
     
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  4. Tombukt2

    Tombukt2 Senior Member

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    The catalyst being below threshold is known to the car by it running a self test on the catalyst system at random times . Unknown times as far as I know . So maybe it runs this test right after you fill up with gas.? I have no idea I have nothing to refer to as to how the test works. I don't know of other things being wrong could fool the system into this particular code.
     
  5. Koich

    Koich New Member

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    It does not have it's original catalytic. About 221K miles on the car, and yes, it does burn oil, but that doesn't explain the CEL on/off with the fuel level.

    Sort of. The CEL goes away after 1 fuel bar is consumed, and comes back when only 3 or 4 fuel bars are left.

    That's an interesting theory.
    But if so, resetting when fueled doesn't make sense either?
     
    #5 Koich, Jul 9, 2025 at 4:19 PM
    Last edited by a moderator: Jul 9, 2025 at 6:59 PM
  6. Tombukt2

    Tombukt2 Senior Member

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    That's not a theory that's what the car does that's how it knows to set that code what do you think it just makes it up?
     
  7. Koich

    Koich New Member

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    You're taking my comment the wrong way. ;)
    My Interesting theory comment was based on your comment of "So maybe it runs this test right after you fill up with gas.?" and respect to your knowledge. :)
     
  8. ChapmanF

    ChapmanF Senior Member

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    The gen 3 repair manual says the test takes about 30 seconds to run and is done once per driving cycle, some time after the engine warms up.

    I don't have a gen 2 repair manual handy right now to look at, but I'd be surprised if it didn't also take the mystery out of when the test runs.