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Edfast702 10/7/2012
2001 Mercury Cougar V6 6 Cyl 2.5L - Engine
My check engine light is on.
I took it into the shop and they told me that it is my catalytic converter because all of the other tests did not pass. Im assuming that is the problem up because I cannot think of anything else.
1 Answer
  • CVO
    CVO 10/7/2012
    The powertrain control module must trigger a code P0420, it may mean that one or more of the following has happened:

    Leaded fuel was used where unleaded was called for
    A damaged or failed oxygen sensor (HO2S)
    Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
    The engine coolant temperature sensor is not working properly
    Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
    Retarded spark timing
    The oxygen sensors in front and behind the converter are reporting too similar of readings
    Leaking fuel injector or high fuel pressure
    Cylinder misfire
    Oil contamination

    Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
    Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
    Inspect the downstream heated oxygen sensor (HO2S), replace if necessary.

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