P0420 and bad oxygen sensors

"Steve Austin" > wrote in message
. ..
> Ray O wrote:
>> "Jeff Strickland" > wrote in message
>> ...
>>> > wrote in message
>>> ...
>>>> On Wed, 18 Mar 2009 16:47:14 -0700, "Jeff Strickland"
>>>> > wrote:
>>>>
>>>>
>>>>>> I would suggest that you don;t know what you are talking about. The
>>>>>> sensor that monitors the catalyst is just a second o2 sensor. If the
>>>>>> second sensor tracks the first sensor, the converter is not
>>>>>> functioning. It's that simple. It is possible, but not likely, that
>>>>>> the conveerter could fail in such a way that the sensor would not
>>>>>> pick
>>>>>> up the failure, but extremely unlikely that the sensor would indicate
>>>>>> a bad cat if it was functioning properly. Both sensors would need to
>>>>>> fail, and in such a way that they did not cause an O2 sensor failure
>>>>>> code, in order for this to occurr. EXTREMELY unlikely, when you
>>>>>> actually know how they work.
>>>>>>
>>>>>> When I say it is POSSIBLE the cat could fail and not be caught by the
>>>>>> sensors, it is because the oxidation catalyst uses up oxygen, and the
>>>>>> reduction catalyst frees up oxygen, but generally not the same amount
>>>>>> each way.
>>>>>
>>>>> I know what the sensors monitor.
>>>>>
>>>>> What I said was that there is no sensor to monitor the sensors. The
>>>>> sensors
>>>>> send false data, particularly in the case of the O2 sensors _because_
>>>>> the
>>>>> environment that these sensors operate in is very hostile. Given the
>>>>> specific code being discussed, P0420, all I'm saying is that the
>>>>> sensor is
>>>>> more likely a failure item than the CAT.
>>>> With OBD2 the sensors are virtually self checking - There ARE
>>>> conditions that can slip by, but they are rather rare. On pre-obd2
>>>> cars a lazy sensor would not be caught - but the computer knows what a
>>>> GOOD sensor should do, and if the signal does not behave the way it is
>>>> expected to, it gets flagged (too few counts, or too narrow a range).
>>>
>>> All of this discussion came about from an article posted that said to
>>> not be too quick to replace a CAT due to a P0420 report. Among other
>>> causes, the author makes clear that the sensors get lazy.
>>>
>>> <quote>
>>> Lazy oxygen sensors can distort the data and confuse the PCM.
>>>
>>> </quote
>>>
>>> That's precisely what I have been saying.
>>>
>>> Unless one has done something to contaminate a CAT, a lazy sensor is
>>> going to report a false condition. In this instance, the after-CAT
>>> sensor will report that the CAT is not doing it's job. I'd be looking at
>>> the sensor first before I'd be ordering a new CAT.
>>>
>>
>> A lazy O2 sensor will not set P0420. P0420 is set when sensor #2 sees a
>> "lively" signal, indicating that the cat is not working. If the cat is
>> working, the signal from sensor #2 would seem kind of lazy, and the
>> "lazy" signal would indicate a good cat.
>
> Different manufactures have different strategies in the pcm to test the
> cat. If the strategy is to drive the exhaust lean/rich or rich/lean and
> measure the time delay between the oxygen sensors, then a lazy front
> sensor could set a p0420/p0430.

True, but in that scenario, the codes were not caused by the post-cat
sensor.
--

Ray O
(correct punctuation to reply)