Check Engine Light (P0420 code) on and then goes off

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Mattdr28, May 9, 2024.

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  1. Mattdr28

    Mattdr28 Junior Member

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    I was driving my 2008 prius yesterday and noticed the check engine light was on. I hooked up the OBD reader and got a code for P0420. Today I was driving to work and noticed the check engine light was off. Catalytic converter was replaced 3 years ago after being stolen and at least one O2 sensor has been replaced in the past. Should I take it in to the mechanic to get checked out or wait for the light to come on again?

    Strangely, my gas gauge hasn't worked for years and then suddenly started working this morning.
     
    #1 Mattdr28, May 9, 2024
  2. ChapmanF

    ChapmanF Senior Member

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    What kind of replacement catalytic converter was put on?

    You might want to edit your title (if you still can) to say P0420.

    If you can't, you can use Report and ask a mod to do it.
     
    #2 ChapmanF, May 9, 2024
  3. Mattdr28

    Mattdr28 Junior Member

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    AE cat converter. I don't know the make
     
    #3 Mattdr28, May 9, 2024
  4. PriusCamper

    PriusCamper Senior Member

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    Your gas gauge, shift indicator & speedometer are known as the combination meter. There's an easy to replace capacitor on that board that's causing that screen to stay. Get that fixed right away... You're lucky it hasn't created problems for you after all this time.

    Unless of course you're saying that the screen worked, but the gas gauge didn't? Regardless that situation is not related to your catalytic converter.
     
    #4 PriusCamper, May 9, 2024
  5. Mattdr28

    Mattdr28 Junior Member

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    It's the latter. The screen worked and gas gauge didn't. But that's a separate issue from the car throwing the code for P0420. Seems like a coincidence. Any thoughts on the check engine light coming on, throwing the P0420 code, and then turning off?
     
    #5 Mattdr28, May 9, 2024
  6. Mattdr28

    Mattdr28 Junior Member

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    Took it to the mechanic this morning. He says its the cat converter. After markets apparently only last 2-3 yrs after which they start developing cavities inside. Only problem is the emission quality might go down. He told me that if the check engine light comes on, the computer throws code P0420, and then the light goes off that means its the Cat converter not the O2 sensor.
     
    #6 Mattdr28, May 10, 2024
  7. mr_guy_mann

    mr_guy_mann Senior Member

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    Having the MIL (check engine light) come on then off means that the catalytic converter test has marginal results. It "barely" fails a couple times, which sets the codes. Then maybe it just passes 2-3 times in a row, which is enough to turn the light off.

    Having an AfterMarket cat is near the top of my list for causes of a P0420. I would still check everything else:

    No exhaust leaks ahead or near any of the A/F or O2 sensors. No misfires. A/F & O2 sensors function "normally" (can report full rich & lean conditions), and total fuel trims are 0 +/- 10%.

    Posted via the PriusChat mobile app.
     
    #7 mr_guy_mann, May 10, 2024
    Brian1954 and PriusCamper like this.
  8. Tideland Prius

    Tideland Prius Moderator of the North
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    #8 Tideland Prius, May 12, 2024
  9. AzusaPrius

    AzusaPrius Senior Member

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    I do not think your aftermarket cat has cavities yet, it would need to be very clogged and overheating to start creating cavities.

    Unless your car is burning lots of oil and clogging a really cheap cat that is poorly made.

    You can always use a bottle of Cataclean.

    I have used this product after my car had P0420 for a year and 30 days after using the product the code went away and I quickly went to smog it and it passed.
     
    #9 AzusaPrius, May 12, 2024
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