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.

Tags:
  1. Koich

    Koich New Member

    Joined:
    Jul 3, 2025
    3
    0
    0
    Location:
    Bay Area
    Vehicle:
    2009 Prius
    Model:
    N/A
    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?
     
    #1 Koich, Jul 3, 2025
  2. Brian1954

    Brian1954 Senior Member

    Joined:
    Jan 31, 2021
    1,787
    701
    0
    Location:
    South Central PA, USA
    Vehicle:
    2011 Prius
    Model:
    III
    Does the car have the original catalytic converter? How many miles on the car? Does your engine burn oil?
     
    #2 Brian1954, Jul 4, 2025 at 9:07 AM
  3. frodoz737

    frodoz737 Top Wrench

    Joined:
    Aug 26, 2010
    4,518
    2,433
    33
    Location:
    Texas
    Vehicle:
    2015 Prius
    Model:
    Four
    ...and this CAT code goes away by itself when you fill up?
     
    #3 frodoz737, Jul 4, 2025 at 10:14 AM
    Brian1954 and Tombukt2 like this.
  4. Tombukt2

    Tombukt2 Senior Member

    Joined:
    Nov 29, 2020
    12,183
    2,150
    0
    Location:
    Durham NC
    Vehicle:
    2009 Prius
    Model:
    Base
    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.
     
    #4 Tombukt2, Jul 4, 2025 at 10:47 AM
  5. Koich

    Koich New Member

    Joined:
    Jul 3, 2025
    3
    0
    0
    Location:
    Bay Area
    Vehicle:
    2009 Prius
    Model:
    N/A
    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

    Joined:
    Nov 29, 2020
    12,183
    2,150
    0
    Location:
    Durham NC
    Vehicle:
    2009 Prius
    Model:
    Base
    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?
     
    #6 Tombukt2, Jul 9, 2025 at 7:38 PM
  7. Koich

    Koich New Member

    Joined:
    Jul 3, 2025
    3
    0
    0
    Location:
    Bay Area
    Vehicle:
    2009 Prius
    Model:
    N/A

    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. :)
     
    #7 Koich, Jul 9, 2025 at 7:47 PM
  8. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    26,411
    17,304
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    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.
     
    #8 ChapmanF, Jul 9, 2025 at 9:58 PM
(You must log in or sign up to post here.)