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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) |
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