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  1. #1
    Forum Overlord xtmesisx's Avatar
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    Default CEL HELP: P0421 (Warm Up Catalyst Efficiency Below Threshold (Bank 1))

    So about 90 miles into my 170 mile trip home from AC, a CEL popped up while cruising at highway speed (~70MPH). I whipped out the trusty ScanGauge and it came back with P0421, or Warm Up Catalyst Efficiency Below Threshold (Bank 1). The car wasn't behaving oddly at all so I cleared the code figuring it would come back if I really had a problem. (Note: I researched the code somewhat for clearing the code.)

    Research seems to indicate it could either be failing/failed cats, failing/failed 02 plugs (post-cat) or fouled spark plugs. I just replaced the plugs and coils <10k miles ago, so I highly doubt it's that. Also, my main cats were replaced around ~18k miles for reasons that are still beyond my knowledge. (Note pt2: I filled the gas tank 30 miles before the CEL tripped.)

    So what do you guys think? I cleared the code and it didn't come back for the rest of the trip (70-80 miles). Obviously I'm going to keep an eye on it, but I think it's just a fluke.
    Need a fender roller? I have one.

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  2. #2
    Forum Overlord xtmesisx's Avatar
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    bump? pros? anybody? LOLs?

  3. #3
    Destroyer of Vehicles tweiss3's Avatar
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    Most likely the precat is done. Happened to mine. You can replace the front cat (approx $800 for just the headers) go after market with an 02 sim, or just go get an 02 sim. The problem with the failed cat is it can break up and be sucked into the motor and cause catastrophic failure. Bad thing is that only the front precat is monitored, so the back one is also probably done too.


    2004 6s -Broken at 114,000 Miles

    1991 RX7 - the new DD, 158,000 miles




  4. #4
    Forum Overlord xtmesisx's Avatar
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    Quote Originally Posted by tweiss3 View Post
    Most likely the precat is done. Happened to mine. You can replace the front cat (approx $800 for just the headers) go after market with an 02 sim, or just go get an 02 sim. The problem with the failed cat is it can break up and be sucked into the motor and cause catastrophic failure. Bad thing is that only the front precat is monitored, so the back one is also probably done too.
    If the cat was shot, don't you think the CEL would have come back sooner? I just drove another 40 miles tonight and filled up again with no issue. Guess we'll see what happens on my 160 mile adventure to class tomorrow. I guess the partially good news is that I still have some warranty left (7yr/100k miles FTW).

  5. #5
    Destroyer of Vehicles tweiss3's Avatar
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    Thats how mine happened, went on and off for almost 3 months, than stayed on solid after that till i replaced it.

  6. #6
    Forum Overlord xtmesisx's Avatar
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    Quote Originally Posted by tweiss3 View Post
    Thats how mine happened, went on and off for almost 3 months, than stayed on solid after that till i replaced it.
    Interesting..... maybe I'll take her off to the dealer for an inspection. It is convenient timing, considering MSDS headers are on sale....

  7. #7
    Destroyer of Vehicles tweiss3's Avatar
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    another thing, if you are under 80k, they have to replace it due to the Federal Emissions, mine went at 86k so i was screwed

  8. #8
    Forum Overlord xtmesisx's Avatar
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    Quote Originally Posted by tweiss3 View Post
    another thing, if you are under 80k, they have to replace it due to the Federal Emissions, mine went at 86k so i was screwed
    I actually rolled 80k about 30 miles before the CEL tripped. I do have the "extended warranty" though, so I may be in good luck (though I'm not counting on it).

    BTW, 350 miles and no CEL.

  9. #9
    Destroyer of Vehicles tweiss3's Avatar
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    Mine had gaps of about 800 miles at first. That extended warranty might be your savior, since doing the swap is a PITA and expensive.

  10. #10
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    I had the same problem with bank two. I installed the o2 sim and it went away. when I pulled the exhaust manifold to put on my headers i looked at the cat and from what I could see it looked fine.
    03 Mazda 6S MSDS Headers, Custom Straight Pipes, O2 Simulator, CAI, EGR Mod,
    Coolant Bypass, Ngk Iridium Plugs, 6000k HID,
    De-badged with Flat Black Mazda Emblems in Grill and on Trunk

  11. #11
    Forum Overlord xtmesisx's Avatar
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    Quote Originally Posted by tweiss3 View Post
    Mine had gaps of about 800 miles at first. That extended warranty might be your savior, since doing the swap is a PITA and expensive.
    Wouldn't you know it.... Almost 800 miles on the nose and this pops up:

    P0421
    @ 80,820 miles
    PID03 LOOP IS OPEN
    PID04 LOD = 35 (Engine Loading)
    PID05 WT *F 191 (Water Temp)
    PID0C RPM 2381
    PID0D MPH 58
    PID0E IGN 41 (Ignition Timing)
    PID0F IAT *F 71 (Intake Temp)
    PID10 GPH 0.00 (Gallons per Hour)
    PID11 TPS 20 (Throttle position)

    Calling the dealer tomorrow to get this sorted out. I'm going to check the 02 sensors first (obviously) because they're the cheapest part to replace. I'll update this thread as info comes along. I may be looking at a nice set of CPe headers SON!

  12. #12
    Website Developer / Sponsor Admin GeneralPatton's Avatar
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    LOL
    2007 Mazda 6i Sport VE MTX
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  13. #13
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    cats bad.

  14. #14
    Forum Overlord xtmesisx's Avatar
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    Quote Originally Posted by katyb View Post
    cats bad.
    No you're not, you're wonderful!

    Seriously though, that's what I'm thinking. I just hope the dealer honors the extended warranty. FWIW, this is the second time I've had problems with the cats on this car.

  15. #15
    Destroyer of Vehicles tweiss3's Avatar
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    Yea, they kinda really suck on this car. And what gets me even more, they only monitor one (the front) precat, WFT?!?

 

 
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