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  1. #1
    ptowle
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    ck engine light

    I had it checked and states secondary oxygen system. anyone know what that is? z3 2.8 260,000


  2. #2
    Robert Platt Bell
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    Secondary Oxygen Sensors


    OBD II cars have two sets of oxygen sensors. The primaries monitor the exhaust before the cat and send a signal to the engine management computer to adjust the fuel injection to maintain the 14.65:1 optimal air to fuel ration (Stoichiometric ratio) known as "Lambda" (hence these sensors are sometimes called "Lambda sensors" particularly on older cars).

    Older cars had these systems, of course. OBD II goes a step further by providing additional sensors downstream of the catalytic converter to monitor the exhaust AFTER the cat has done its job. These send signals to the engine management computer for emissions data logging, which in turn is read by the emissions inspector.

    There are a number of failure modes and a number of different error codes. Without knowing the exact code, it is hard to tell. If the CE light is reset prior to testing, it may give a "sensor not ready" signal.

    A hard error, such as a disconnected sensor or a broken sensor wire will set off the CE light and will appear immediately after the CE light is reset.

    Other errors take time to appear, such as "Catalytic Converter Efficiency Below Threshold" - which may be caused by the primary sensors, secondary sensors, or the cat - or even other causes.

    The sensors are good for 100,000 miles on OBD II cars. If yours are even near the replacement point, I would just replace them.

    The 2.5 (e.g., 323) and 1.9 models have single sensors for both primary and secondary systems. The 2.8, 3.0, and 3.2 have two sensors for both primary and secondary (monitored as banks 1-3 and 4-6).

    Pelican Parts has the sensors for about $108. Use good quality "plug and play" sensors, not some generic job that you solder in. Walk away from crappy parts, they just cause frustration when they don't work - and you end up chasing your tail.

    Good Luck.

  3. #3
    ptowle
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    Re: Secondary Oxygen Sensors

    thanks Robert,
    I checked further into it with your good info. light did not come back for a week and it is the secondary air system and with loiw flow. Sounds like maybe the catalytic converter is gone.

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