Thread: Diagnose P0420 - HPT Scan Link

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  1. #1 Diagnose P0420 - HPT Scan Link 
    GTP Level Member Impala131's Avatar
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    Click Here for Scan

    Have the very common P0420 scan on the new Impala I bought. I'd appreciate it if some of you had a look at this scan with HPT and give me an educated opinion on the cause of the problem. I added the downstream O2 sensor to the main table so both O2 sensors would be logged.

    Car is a 2004 Impala LS (N/A 3800SII)

    The huge KR spike at WOT might be from having a bad motor mount plus a dogbone mount that's cracked in two. Motor moves around more than it should.


    Thanks in advance.
    2003 Grand Prix GTP.... the journey begins again....
    E85 Self Tune with HPT, 80# Injectors, Gen V 3.4 pulley, SD Headers, AL605 Plugs
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  2. #2 Re: Diagnose P0420 - HPT Scan Link 
    Turbo is the way to go. BillBoost37's Avatar
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    Please read up on the 420, it's most likely caused by a bad or aftermarket cat.
    I drink..so consider that when reading my posts.

    2010 Audi A6 Dual IC's
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  3. #3 Re: Diagnose P0420 - HPT Scan Link 
    GXP Level Member 91parkave's Avatar
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    You only need to log bank 1 sensor 1. Bank 1 sensor 2 is only to monitor catalytic performance.
    06 GXP | 222/227 cam/cartuning turbo kit on 8psi/meth/e85 coilovers/ still on stock trans at 130k
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  4. #4 Re: Diagnose P0420 - HPT Scan Link 
    GTP Level Member Impala131's Avatar
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    Quote Originally Posted by 91parkave View Post
    You only need to log bank 1 sensor 1. Bank 1 sensor 2 is only to monitor catalytic performance.
    What I am trying to establish, is if the catalytic converter is indeed the culprit, or a lazy/rogue O2 sensor is giving false readings. You are exactly right, the downstream O2 monitors catalyst efficiency... that's why I want to verify that the sensor is working properly.

    I understand that more than likely, it's a bad cat... but I just want confirmation that it's not an O2 issue before I spend the money replacing the cat.
    2003 Grand Prix GTP.... the journey begins again....
    E85 Self Tune with HPT, 80# Injectors, Gen V 3.4 pulley, SD Headers, AL605 Plugs
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  5. #5 Re: Diagnose P0420 - HPT Scan Link 
    GXP Level Member 91parkave's Avatar
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    Itd be hard regardless to tell which one because of the lack of exhaust flow due to a j a cred up cat would cause lazy oscillations on the chart display as well as a lazy 02 sensor wI'll cause the same issue.
    06 GXP | 222/227 cam/cartuning turbo kit on 8psi/meth/e85 coilovers/ still on stock trans at 130k
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  6. #6 Re: Diagnose P0420 - HPT Scan Link 
    GTP Level Member Impala131's Avatar
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    Quote Originally Posted by 91parkave View Post
    Itd be hard regardless to tell which one because of the lack of exhaust flow due to a j a cred up cat would cause lazy oscillations on the chart display as well as a lazy 02 sensor wI'll cause the same issue.
    Absolutely agreed, I was just hoping someone would be able to look at the scan and saw or understood something that would be a giveaway.
    2003 Grand Prix GTP.... the journey begins again....
    E85 Self Tune with HPT, 80# Injectors, Gen V 3.4 pulley, SD Headers, AL605 Plugs
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