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Thread: New to the MPs scene need help with code po421

  1. Default New to the MPs scene need help with code po421

    Hey guys,
    First a big hello
    I just got a 2006 Mazda mps 6. About two weeks ago
    And today the engine light came on lucky the car came with reader so found the code po421 looked it up either cat,o2 sensor or spark plug.
    I did notice last two days after a cold start been getting a little miss hard to notice but idles fine and goes away quick
    Sso advice do I look at replacing coil and spark plugs first
    On furthur research comes up saying 99% is the cat
    Thanks any help appreciated

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    Does it have a custom exhaust or all stock standard?

    Sent from my Nexus 5 using Tapatalk

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    Not sure I think it's standard didn't get much history with it has a hyper access machine to change tunes has had some work done has a after market psi gauge not sure about turbo as never seen another one that compare too

  4. #4


    Grab some pics of the engine bay and downpipe so we can see what's been done. Could definitely be an aftermarket downpipe.

    Sent from my Nexus 6 using Tapatalk

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    hope this helps upon looking underneath car cant see where the cat is
    Looks standard exhaust muffler end the other end possibly been replaced ?
    Thanks for any help guys my first ever Mazda and a first ever turbo
    Last edited by Robceline; 30-05-2016 at 03:18 PM.

  7. Default

    Looks Like stock DP and turbo to me. Nice intake but. Is you Hyper access able to do MAFCAL?

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    Havent tried it yet only plugged in to find fault code never used one before comes up with diffrent tune set ups put don't want to play with till I know what I'm doing I got it from a lady she got it from her ex which had done work on it was told had air intake short shifter and was told 230 kws which I can't see how just had timing chain done so I take it the cat is in the down pipe I think the mafcal has already been tuned from what I have seen on Google has maf options
    Last edited by Robceline; 30-05-2016 at 09:55 PM.

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    Still trying to get a answer on the code error have reset it two days ago has not come back

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    Hypertech 62003 max energy sport saver Mazda is what I have

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    The code your getting suggest that it is a faulty cat, but you said you haven't seen it in 2 days after a reset. It could be a bad fuel.
    Power wise, its pretty good.

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    Yeah been two days no fault I do get a little miss now and then only when cold hardly notice it

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    Problem solved well a stupid p plater just wrote my parked car off so the shortest car I have ever own three weeks
    Looking at another one possible next week

  14. #15
    Join Date
    Apr 2010



    And RIP another 6

  15. #16
    Join Date
    Apr 2014
    auckland NZ


    part out?

  16. #17


    Sent from my Nexus 6 using Tapatalk

  17. Default

    Insurance took car look out of at auction looking at a car in Canberra next weekend

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