Question about 2000 Mazda Protege

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Determine which O2 sensor to replace.

Need help to determine which O2 sensor to replace on a '00 Mazda Protege. One mechanic said the after cat sensor, and another said the before. How can I be sure which one.

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  • somz Feb 10, 2009

    It is a 1.6L 4 cylinder. The code is P0171. Thank you for your time.

  • somz Feb 10, 2009

    Thanks a lot for your help. Will go the mechanic today and get it checked thoroughly. One of them apparently did a diagnosis and came up with the after cat o2 sensor theory, but i just want to be sure. Thanks a million.

  • DLGONZALEZ Mar 13, 2009


  • Dave Real
    Dave Real May 11, 2010

    What codes turned on the check engine light? There are different codes for the front or rear one.


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  • Master
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There are many reasons for a PO171 code, these need to be checked before replacing O2's:

  • Vehicle driven low on fuel or until it ran out of fuel
  • One or more injectors restricted or pressure regulator has failed
  • Fuel delivery system supplying too much or too little fuel during cruise or idle periods (e.g., faulty fuel pump, or dirty fuel filter)
  • Air leaks after the MAF sensor, or air leaks in the PCV system
  • Air leaks at the EGR gasket, or at the EGR valve diaphragm
  • Exhaust leaks before or near where the front HO2S is mounted
  • HO2S is contaminated, deteriorated or it has failed
  • Fuel control sensor is out of calibration (i.e., ECT, IAT or MAP)
  • Base engine "mechanical" fault affecting one or more cylinders.
  • Posted on Feb 10, 2009

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    • Mazda Master
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    What engine and what code do u have, after cat means the sensor that is located behind the catalytic converter in the exhaust system, but if the code definition says: catalytic converter efficiency below limits, u may need a new catalytic converter.

    Posted on Feb 10, 2009

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