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Edfast702 10/7/2012

2001 Mercury Cougar V6 6 Cyl 2.5L


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 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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