p0304,p0400,p0420 cause related?

Discussion in 'General Motoring' started by tedt, Feb 12, 2004.

  1. tedt

    tedt Guest

    I recently bought a '96 Legacy GT with 195,000 miles
    from a friend. It has a history of the dealer changing
    the EGR valve. It is now showing the codes P0304 (
    cylinder #4 misfire) P0400 ( EGR flow malfunction) P0420
    (catalyst system efficiency below threshold). It runs
    good but mpg and pick-up seem a bit low. My question
    is if these could be related and what problem should
    I tackle first.

    Thanks,
    Ted
     
    tedt, Feb 12, 2004
    #1
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  2. At this mileage you're probably due for the 2nd possibly 3rd front (at
    least) O2 sensor. Possibly new cat converter - some mechs have a
    pressure test for the cat using the O2 bung hole.

    Not clear in your post, but if the other codes come back immediately
    after clearing them they could be real second and 3rd issues, but I have
    read about bad cats causing misfires/pinging/loss of power etc.

    If you want a lot of info and help on tackling these problems yourself
    check the forums at www.usmb.net

    Carl
    1 Lucky Texan
     
    Carl 1 Lucky Texan, Feb 13, 2004
    #2
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  3. tedt

    TG Guest

    The "low catalyst efficiency" is usually caused by air entering the exhaust
    system where the headers bolt to the head (exhaust manifold). Buy new
    gaskets, studs, and nuts and watch the torque. You might be able to get away
    with just gaskets but chase the threads on the studs and nuts if you do it
    that way otherwise the torque readings will be insufficient to seal things
    up and additional torque will break a stud. TG
     
    TG, Feb 15, 2004
    #3
  4. tedt

    GTT Guest

    err, I see this is a fairly old post, but could you explain "chase the
    threads ..."
    for this old feller?

    Thanks,
     
    GTT, Feb 26, 2004
    #4
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