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  1. #1
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    On & off "Check Engine" light. P0420

    I guess it's too much to hope for a Z3 to run trouble-free forever. =(

    Well, in recent weeks my Check Engine light has been going on and off, at almost predictable intervals. After the first time the light came on (while I was driving the car), it dissappeared after about 200 miles (also when I was driving). It came on again after another ~200 miles, and off again. When it turned on again, I finally got the fault code read, and it was the following:

    P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

    So I went ahead and searched this board for into. Now my Z3 is a 96, with 77K miles on it. So I wouldn't be surprised if the cause of all this is a deteriorating oxygen sensor (the front one), so replacing the sensor would be the simplest thing to do. Now I found posts that say the sensor might just be lazy, and if so, how do I check a sensor and "whip it into shape"?

    I've also seen RADesign's post that says it's his old spark plugs which was causing the check engine light. I've replaced my spark plugs about 7 months ago, so that shouldn't be an issue, but I'll leave that as a possibility.

    As always your thoughts are appreciated! =)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    1996 BMW Z3 James Bond Edition

    Licensed to Thrill


  2. #2
    Giznaz
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    Replace at least the front O2 sensor

    Don't get sucked into the "bad cat" mentality UNTIL you've driven on new sensors for a bit.

  3. #3
    gbsterling
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    Re: On & off Check Engine light. P0420

    The problem could be the catalytic converter. I had the same symptoms, at almost exactly the same mileage, in my '96 1.9. There was a recall of the 97 1.9s for a converter problem. I think the '96 has the same converter, but it was not recalled as far as I know. There is an 8 year, 80K mile warranty on emissions that chould cover the cat replacement.

  4. #4
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    Get it in quick to see if you can get a free cat.

    Replaceing the font ox sensor can help sometimes but the cat is often a bit tired by now.

  5. #5
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    Re: On & off Check Engine light. P0420

    Am I still covered by that warranty? It's been 10 years since 96. Also, I'm not the original owner of this car.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    1996 BMW Z3 James Bond Edition

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  6. #6
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    Take it to dealer?

    Are you referring to the warranty on the car? Is there some 10 year warranty that comes with every 96 Z3? That seems too good to be true. I don't know since I'm not the original owner.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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  7. #7
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    Re: Replace at least the front O2 sensor

    Yeah, I didn't even consider replacing the cat until people suggested I can get one for free.

    As for the O2 sensor, other than replacing it, is there a way to fix it so it won't be lazy?
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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  8. #8
    Giznaz
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    No way to fix it. It is a consumable item.


  9. #9
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    Yep, it is like an 80,000 miles warranty

    on emissions stuff. You might be over borderline on age but sometimes they will work with you. It won't hurt to try.

  10. #10
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    Most often it's a bad sensor

    The BMW computer is good at thinking it's a bad cat when it's often a bad O2 or Mass Air Flow Sensor. Need4Spd
    Current: '01 M5/'05 M3 Conv. Past:'99 M Roadster/'95 M3

  11. #11
    gbsterling
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    Re: On & off Check Engine light. P0420

    Sorry, you're right, the warranty ran out after 8 years. Time flies when you're having fun, especially in a Z3!

    I would still suspect the catalytic converter though, because of the on and off nature of the problem. It's not out of the question for one the O2 sensors to be at fault, but the recommended change interval for them is 100K miles. I just did mine at about 112K. My check engine light did the same thing, and I think it was more likely to come on when I was driving on back roads, because the lower engine speeds and exhaust flows wouldn't allow the converter to get up to temperature. Once I had the converter replaced the light never came on again.

  12. #12
    PTOWLE
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    Re: mine lights up if I overfill fuel then goes of

    off within 200-300 miles or 2-3 days. this only happens when a fuel attendant tries to round up the purchase.

  13. #13
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    no such luck

    Dealers were strict on the warranty expiration date. No freebies for me. Oh well, at least now I know all cars have a 8 yr 80Kmile federaly-imposed warranty on the catalytic converter. Possibly useful for my next car.

    I'll just go ahead and change the O2 sensor.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    1996 BMW Z3 James Bond Edition

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  14. #14
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    Yes | No

    It would have been too nice!


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