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Emissions failure with after market exhaust

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    • Spectator

    Hey guys,

    Wondering if any of you have experienced this issue.  I have a full Cobalt exhaust on my NB Miata.  There are no check engine lights but it failed emissions.  The emissions tech said the computer was having an issue getting a cat reading.  In my past experiences with other cars this would usually throw a check engine code.  Has anyone experienced this?  Should I start by replacing the o2 sensor?  Looking for some guidance to get this to pass.

    Thanks in advance,

    Chris

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    • Ace
    • ★★★★★

    What year is your Miata?

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    • Spectator

    It’s a 2004 Miata.

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    • Ace
    • ★★★★★

    If everything is in proper working order, a Cobalt CAT back should make no difference…all the important stuff is before that, if I am not mistaken

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    • Spectator

    I have the full exhaust…headers and everything back.

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    • Novice
    • ★★

    Hook up a scan tool and check for ant stored or pending code.

    Check to see if all monitors are reported as “ready”. You may see the catalyst shown as not supported. This can be due to the cat monitor has not yet run and reported status.

    Check live stream data for both upstream and downstream O2 sensors. Upstream should be moving up/down constantly and the downstream should be steady and relatively flat.

    If it’s a matter of the cat monitor not reporting ready, it’s likely a case where the “drive cycle” the car needs to see to run the monitor checks hasn’t been completed yet. If there isn’t any underlining issues, you just need to drive the car more to complete the “‘drive cycle”

    Drive Cycle:  https://forum.miata.net/vb/showpost.php?p=5904032&postcount=2

    Good luck

     

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