P0420 Code
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Northstar Engines and System Technical Discussion Discussion, P0420 Code in Cadillac Engine Technical Discussion; Ok my car kept throwing the P0420. After futher investigation I found out that the CAT O2 sensor was bad. ...
  1. #1
    STSMAN is offline Cadillac Owners Member
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    P0420 Code

    Ok my car kept throwing the P0420. After futher investigation I found out that the CAT O2 sensor was bad. I replaced it. But didn;t reset the code yet. My question is do I reset the codes or will it reset itself with a few drive cycles?? if I remember correctly that code will not reset right away unless you clear it manually. Is my memory serving me well ??

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    Eldyfig's Avatar
    Eldyfig is offline Cadillac Owners Connoisseur
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    Re: P0420 Code

    Yes, it does serve you well. Even when a code is posted, the PCM still tests that particular sensor. When the sensor gives passing parameters, the code will remain in history for a particular number of ignition cycles before it disappears.

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    mcowden is offline Cadillac Owners Connoisseur
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    Re: P0420 Code

    Quote Originally Posted by Eldyfig
    Yes, it does serve you well. Even when a code is posted, the PCM still tests that particular sensor. When the sensor gives passing parameters, the code will remain in history for a particular number of ignition cycles before it disappears.
    Exactly as Eldyfig said, the code will stay in memory as a "history" code, meaning that the code happened in the past and is not currently a problem. It won't hurt anything to leave the code in there. As long as the sensor is replaced, you are back to normal.
    Mike

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    STSMAN is offline Cadillac Owners Member
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    Re: P0420 Code

    Quote Originally Posted by mcowden
    Exactly as Eldyfig said, the code will stay in memory as a "history" code, meaning that the code happened in the past and is not currently a problem. It won't hurt anything to leave the code in there. As long as the sensor is replaced, you are back to normal.
    I understand about the code in history, ,however what about the MIL light?? Will it clear after a few drive cycles?? It should clear when that code becomes a history code.

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    mcowden is offline Cadillac Owners Connoisseur
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    Re: P0420 Code

    Quote Originally Posted by STSMAN
    I understand about the code in history, ,however what about the MIL light?? Will it clear after a few drive cycles?? It should clear when that code becomes a history code.
    If the SES light is still on, you may have a CURRENT code in there in addition to the HISTORY P0420. Have you checked for codes in every category, or just waited for all of the categories to scroll by? If there are no other codes, then clearing the P0420 will snuff the SES light. If you do find another code, let us know so we can help you figure it out.
    Mike

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    STSMAN is offline Cadillac Owners Member
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    Re: P0420 Code

    Quote Originally Posted by mcowden
    If the SES light is still on, you may have a CURRENT code in there in addition to the HISTORY P0420. Have you checked for codes in every category, or just waited for all of the categories to scroll by? If there are no other codes, then clearing the P0420 will snuff the SES light. If you do find another code, let us know so we can help you figure it out.
    no just the current P0420 code.. but mind you I just repaired the car today.
    I need to complete more drive or ignition cycles. The light should go out after 3 of these cycles, we will see.

    Thanks for the help

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    mcowden is offline Cadillac Owners Connoisseur
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    Re: P0420 Code

    Quote Originally Posted by STSMAN
    no just the current P0420 code.. but mind you I just repaired the car today.
    I need to complete more drive or ignition cycles. The light should go out after 3 of these cycles, we will see.

    Thanks for the help
    Actually I just looked up the P0420 code and it means "Catalyst System Low Efficiency," which means that the catalytic converter isn't working as it should. Which O2 sensor did you replace? The one before or after the converter? It's possible that the converter is bad and will need to be replaced to make the SES go away. If the code still shows up as CURRENT, the problem is still happening.
    Mike

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    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: P0420 Code

    If you do not clear it, it will extinguish the MIL and go into history after 3 ignition cycles with no fault detected. It will then go into, and stay in, history for 50 ignition cycles, after which it will be deleted.

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    ewill3rd is offline Cadillac Technician
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    Re: P0420 Code

    You'll probably be looking at that light for awhile unless you clear it.
    As Ranger said it must pass the diagnostic test 3 times before the light will go out. The catalyst monitor test might not run every time, in fact it may actually rarely run.

    Problem number two is... a bad O2 sensor will not set a P0420. A bad O2 sensor will set a bad O2 sensor code. P0420 indicates that the O2 sensors are in fact good and the catalyst is not doing it's job. The O2 sensor diagnostic tests MUST pass before the P0420 DTC diagnostic will even run. If the PCM knows the O2 sensors are operating properly and the test enable criteria are met the P0420 test will run.


    These conditions must be met in order for the diagnostic to run:
    No TP sensor, MAF sensor, MAP sensor, IAT sensor, ECT sensor, HO2S, HO2S Heater, Fuel trim, Misfire, EVAP System, EVAP Control, VSS, or Idle Speed DTCs are set.

    • Engine speed since end of last idle period is more than 800 RPM for 46 seconds.

    • Idle period is less than 180 seconds (3 minutes).

    • Actual engine speed is within 100 RPM of desired engine speed (Non-California)

    • Actual engine speed is within 125 RPM of desired engine speed (California).

    • Engine run time is more than 7.5 minutes.

    • The predicted catalyst temperature is between 420°C and 650°C (788°F and 1202°F).

    • BARO is more than 75 kPa.

    • ECT is between 71°C (160°F) and 120°C (248°F).

    • IAT is above -6°C (21°F).

    • Engine is in closed loop fuel control.

    • Tests attempted this ignition cycle is less than 12.

    • Test attempted during idle period is less than 1.
    Here are the conditions for the code to clear or the MIL to go off:

    The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.

    • The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

    • The DTC can be cleared by using the scan tool Clear DTC Information function.

    HTH

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    prcouture is offline Cadillac Owners Member
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    Re: P0420 Code

    Read these details and had the cat replaced on my '00 Deville after the P0420 code came on. Code returned! Is it possible one of the O2 sensors crapped out when the cat was bad? Curious if anyone had success fixing the P0420 code with a replaced O2 sensor.
    Thanks

  12. #11
    krimson_cardnal's Avatar
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    Re: P0420 Code

    prcouture - Re-read what ewill just posted carefully. P0420 is not an O2 sensor code. In fact if thrown it means the O2's are good. K_C

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    Submariner409's Avatar
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    Re: P0420 Code

    eWill3rd posted that ^^ 2 1/2 years ago...........I believe that you have an after-cat O2 sensor which monitors the after-cat oxygen content, a measure of cat efficiency. If that sensor was removed and replaced during the cat exchange it's entirely possible that an 8 year old wire, subject to some intense heat, has let go. The instructions for that sensor also caution about bending one of the wires which carries a "clean air reference" signal. That after-cat sensor can be found at Lindsay Cadillac over there >>>>> (Parts, Luke) or www.rockauto.com. It is a plug & play item, up on a lift.

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    krimson_cardnal's Avatar
    krimson_cardnal is offline Cadillac Owners Enthusiast
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    Re: P0420 Code

    prcouture Reread what ewill posted carefully. A P0420 code being thrown actually means the O2's are good. A bad O2 will throw an O2 code. K_C

    oops - didn't mean to say that again - looks like sub and me crossed paths

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    Re: P0420 Code

    Quote Originally Posted by prcouture View Post
    Read these details and had the cat replaced on my '00 Deville after the P0420 code came on. Code returned! Is it possible one of the O2 sensors crapped out when the cat was bad? Curious if anyone had success fixing the P0420 code with a replaced O2 sensor.
    Thanks
    Hmmmmmmmmmmmmmmmmmm

    First off did you have a GM converter installed?
    Was it one of those aftermarket converters that is 1/2 the size?

    The GM catalytic converter, P/N 25755681

    Some will also require a new software calibration to keep the converter from failing again!
    New calibration to correct the EST control circuit in the PCM from turning off spark intermittently, which causes misfire events, possibly damaging the catalytic converter, as indicated by DTC P0420. Improved EVAP diagnostics and correction for Mode 6.

  16. #15
    ewill3rd is offline Cadillac Technician
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    Re: P0420 Code

    If you bought an aftermarket cat, take the car back and have them install the right one.
    Legally they have to provide you with one that will do the job the GM one did. Many aftermarket ones have trouble meeting the requirements to pass the on board diagnostic tests... which means they don't work.

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