Thread: Po420 help

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  1. #1 Po420 help 
    SE Level Member KCprix's Avatar
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    2007 Pontiac Grand Prix 3.8L base model L26. I've been fighting this code multiple times for better than a year. The car runs/drives good, but the SES light triggers occasionally always with this code.

    The cat was replaced with OReilly model direct fit after a crack/leak was found.
    I've recently done full tune up. E3 plugs and ac delco wires. I've cleaned the MAF. And keep up on all maintenance and repairs.... I've actually had this cat warrantied 3x now from oreilly.

    The cat does not glow red hot nor does the car seem bogged down or cat clogged in any way.

    I've got a full diagnostic scanner, if anyone can give any advice on specific numbers/ranges to look for???

    No misfires or anything to really indicate a problem....

    So i would like advice on what to look for/confirm via testing with scan tool to prove/disprove the cat condition before requesting another warranty? .... Do i keep getting bad after market cats and just need to spend more on a dealer cat?

    What should mV and ranges etc be?

    There has got to be a better way to verify if cat is defect or something else

    ALL HELP AND ADVICE IS APPRECIATED!
    2007 Pontiac Grand Prix (base) 3.8L N/A L26

    THX
    KCprix
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  2. #2 Re: Po420 help 
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    Did you change the O2 sensors?
    GPONA #1 Cyber Bully
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  3. #3 Re: Po420 help 
    SE Level Member KCprix's Avatar
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    Yes, i meant to come back and post that last night.... Both o2 sensors were replaced about 6 months ago with Bosch sensors.

    I've also cleaned the throttle plate/body about every other oil change.

    If i occasionally plug in scan tool.... It usually has said there is a pending po420 code.... And eventually sets the light/code on.

    So there is something going on, she doesn't like.

    As others have mentioned, im wondering if i just need a dealer cat.... I never had any SES lights until aftermarket cat from O'Reilly was installed.
    And OE was only replaced due to a crack in mounting flex flange causing a minor leak.
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  4. #4 Re: Po420 help 
    GrandPrix Junkie
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    Are the E3 plugs platinum? If so, pull those out and stick a normal set of NGK iridium plugs in it. These ignition systems hate plats, just in case you weren't aware.

    As far as the cat, could be too high flow or low cell count and it's tripping the light. I know Walker seems to have good luck as far as a replacement cat goes, as does OEM. Not sure what brand of cat Oreilly's sells.
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  5. #5 Re: Po420 help 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Space the rear o2 sensor out of the exhaust some with the spark plug non fouler trick.

    Aftermarket cats just don't have the cell count and flow too much compared to a stock AC delco converter.

    SMGPFC Member #1
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  6. #6 Re: Po420 help 
    GTX Level Member wstefan20's Avatar
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    I would look at your ltft and stft (long and short term fuel trims) to see if you have an underlying issue such as a vacuum leak. If the fuel trims are within 5%+- while driving, then your options are to 1.) replace with a stock oem catalytic converter 2.) ignore the code 3.) have the code tuned out with hptuners 4.) blow up and/or sell your car

    Good luck!
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  7. #7 Re: Po420 help 
    SE Level Member KCprix's Avatar
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    According to the receipt and comparing that p/n (20205) to OReilly website, it is a Magnaflow cat. Direct fit.

    The e3.44 spark plugs per their site does not say anything about being platnium or iridium.

    @fivefingerdeathpunch tell me more about this non-fouler trick.... How, what do i need and can you link to any other posts on this?
    Is it permanent, if it doesn't help?

    And
    @wstefan20 I've seen fuel trim numbers in my scan tool before, but can you be more detailed on what to look for? As my scan tool doesn't record. But i can look at all the data stream.... Examples of good vs bad numbers please?

    My scan tool is a "Launch Creader VIII"

    although i think if there was a vacuum leak, there would be some kind of driveability issue, there is none... Have done the old school trick of spraying carb cleaner around intakes and gaskets and such... Looking for idle changes.... Again none found.
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  8. #8 Re: Po420 help 
    GrandPrix Junkie
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    Per Oreilly's website:

    Product Information


    • Warranty: 5 Year Limited Warranty
    • Unit of Measure: Each
    • UPC: # 894489000442
    • Hex Size (In): 5/8 Inch
    • Thread Size: 14mm
    • Center Electrode Design: Standard
    • Thread Reach (In): 11/16 Inch
    • Preset Gap (In): 0.04 Inch
    • Short Style Spark Plug: No
    • Resistor Plug: Yes
    • Center Electrode Tip Material: Nickel
    • Ground Electrode Tip Material: Nickel
    • Center Electrode Core Material: Copper
    • Seat: Tapered


    Looks like those plugs will work for now. Check the long term fuel trim and see if it's 16.4%+ at idle in park. If so then you have a vac leak somewhere. Suspect the EGR tube and injector O-rings.
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  9. #9 Re: Po420 help 
    GTX Level Member wstefan20's Avatar
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    Fuel trims usually read as a percentage plus or minus to indicate rich or lean condition and normally I run plus or minus 1-2% depending on acceleration or deceleration. Anything between 5-10% won't cause a drivability issue or check engine light for fuel trims, but could cause your code.

    The carb or propane trick works for large leaks but not small ones. I've got a good post on here about a diy smoke machine that works wonders, but again, only if your fuel trims are off.

    Warning about the spark plug non Fowler trick. Yes, it works, but over time, water condenses around the o2 sensor and corodes it. It works for a quick fix, but not a long term solution.
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  10. #10 Re: Po420 help 
    SE Level Member KCprix's Avatar
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    Thanks for the help all!
    This project will be on a short hold (hopefully short).
    I hit a deer the other night.... Car still runs and drives, no injuries and it could have been much worse but visually Bambi damaged the hood, bumper and headlight.... I still need a body shop to look it over.

    Just wanted to ask everyone to stay tuned.... As after this mess is over, i will get those fuel trim numbers and revisit this.

    Thank you for your help.... Please stay tuned, I'm sure I will have more questions!
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  11. #11 Re: Po420 help 
    SE Level Member KCprix's Avatar
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    Hey again all...with everything going on in the world, I nearly forgot about this post.
    So I did get the body work done from the deer accident...new hood, bumper and 1 headlight assembly...If memory serves around 3k in damage....state liability insurance only, so all of it out of pocket. Had to do it, so I could get back to work.

    Sorry, I never did get around to checking those FT numbers....O'reilly's said they would give me a full refund on their converter, which was ordered thru their commercial guy...When i told him of all the problems, he dug a little deeper and found they actually have 2 direct fit converters, which he described as one having more of the precious metals than the one he originally sold me...TOO BAD HE DIDN'T KNOW THIS 3 WARRANTY EXCHANGES AGO...They shouldn't even be allowed to sell a substandard converter IMHO and I am really surprised a popular company like magnaflow would manufacture this crap.

    Anyway, I did go ahead and get the dealer converter for like $1200, I had no core to return because the original was sold to a recycling yard and as I said, the oreillys one was returned. So I had to pay the core charge.

    And I am (hopefully) happy to continue reporting...NO MORE CHECK ENGINE LIGHT! for nearly a year now. In the past , I had become scared to say that...because after 3 warranties, every time i thought the problem was fixed in the past...A few days later that ****in Murphy guy would show up and turn the light back on.

    Its too bad I didn't get any weed with a po420 code

    Thanks again to everyone who had input on this problem....I thiiiinnnnk it is finally fixed?

    Cheers - Stay safe and covid19 free ALL!
    2007 Pontiac Grand Prix (base) 3.8L N/A L26

    THX
    KCprix
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  12. #12 Re: Po420 help 
    GrandPrix Junkie SgtMarshal's Avatar
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    do they test for emissions in your state? you could have bought a tuner and tuned out the code for less than half of what you paid for an oem converter.
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  13. #13 Re: Po420 help 
    SE Level Member KCprix's Avatar
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    No emission testing in my state...And I wanted it fixed right, after all I went thru, I just wanted it done and over with. Not sure about the future, but I was an Uber driver prior to covid19, and if the tuner didn't work, i saw it as just delaying the inevitable and spending more money....Drunk uber riders will do/say/lie about anything to get a free ride...And all someone had to do was take a picture of the SES light and get me deactivated....claiming my car was unsafe....Uber never has the drivers back...So yes, it might have worked, but just not worth it.
    And as wstefan20 (hi neighbor) mentioned, if it was a fuel trim issue and/or had an unknown vacuum leak (for example)...it just would have been more money wasted.
    2007 Pontiac Grand Prix (base) 3.8L N/A L26

    THX
    KCprix
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  14. #14 Re: Po420 help 
    GT Level Member mguzzo's Avatar
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    did I just read that you spent $1200 for a new downpipe and converter? You could have gotten a new OEM style premium DP and cat off RockAuto for less than a third of that
    --
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  15. #15 Re: Po420 help 
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    Quote Originally Posted by mguzzo View Post
    did I just read that you spent $1200 for a new downpipe and converter? You could have gotten a new OEM style premium DP and cat off RockAuto for less than a third of that
    ...and gotten an HPT to tune and monitor the situation for peace of mind.
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