Wrapper

We've had this discussion before, and I think this is the point that I was
getting wrong.

Our discussion was in regards to P0420, Catalyst System Efficiency Below
Threshold (Bank 1), where I insisted that this meant the after-CAT sensor
had failed. My position was that this code said that whatever was going into
the CAT was good, but what came out was expected to be different, but
wasn't.

During the discussion, I looked for the P0136, but did not recognize it as
the code for the after CAT sensor. I submit that you were telling me that
this code was the one that would come up under the conditions we were
talking about at the time, and I argued against you. Now I see what you were
saying, and I accept your argument by the way.

In layman's terms, can you explain why the after CAT sensor can generate two
different codes in entirely different code families? Most codes are grouped
by family, and there might be failures of a component that can appear as
different codes within the family, but in this instance there are multiple
codes that a single component can generate that are not in the same code
family.

I come from a troubleshooting background, and have over 20 years in various
kinds of troubleshooting. Admittedly, my trade experience is not automotive,
but troubleshooting is troubleshooting until one needs to delve relatively
deeply into the affected systems. My point is, the engineers that I've
worked with have a descernable method to their collective madness, and
having the same component spit out a P0136 or a P0420, depending on
variables I don't fully understand is a bit confusing.

I'm asking if you can help me get my arms around the variables. I have a
good grasp of what the codes are telling me, and I'd be looking at the same
part(s) if either of these two codes came up, but I don't understand why the
code families are so far removed from one another for what is in essence a
failure report of the same component. Well, one report says (without getting
bogged down with detail) the sensor isn't working, the other says the part
that the sensor is looking at isn't working.



BY WAY OF EXAMPLE
The Evaporative Emissions Control System produces a range of codes that are
all within a family. A poorly installed gas cap can give a "system
malfunction" code, a "system gross leak" code, and a "system minor leak"
code. I concede the point that there can be other problems that give these
codes, but my point is that all of the problems that I know about relative
to the system all generate a grouping of codes in the OBD II system. Several
components within the system give a code that is in the family. Relative the
CAT, it appears that there is one component that can give a code that is in
at least two families.






"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
news:a196d$45c2c087$47c2b532$28144@msgid.meganewsservers.com...[color=blue]
>
> "yotaman" <randyvp@gmail.com> wrote in message
> news:1170385012.255537.212770@q2g2000cwa.googlegroups.com...[color=green]
>> I've got a 98 4cyl 4Runner 4x4 which I've recently pulled off a P0136
>> code. From the information I've gathered it is the oxygen sensor
>> located after the cat. I've cleared the code and I'm waiting to see
>> if it comes back. I've read however that bad readings from the 2nd
>> sensor could indicate problems with the 1st sensor at the manifold as
>> it has more influence on fuel/air ratios... even though it isn't the
>> one generating the codes. The truck has 170000 kms and the mileage
>> isn't as good as it used to be, but it seems to be running fine in all
>> other regards. Does it make sense to replace these sensors just as a
>> part of routine maintenance? (they are still original)
>>
>> Curious as to what others have experienced. Thanks in advance.
>>
>> Randy
>>[/color]
>
> If you have a volt meter, with the engine warmed up, rev the engine to
> between 2,000 and 3,000 RPM and check voltage output from Bank 1 Sensor 2
> O2 sensor. It should fluctuate from less than .4 volts to more than .5
> volts. If it doesn't, the O2 sensor is bad. If it does, check for an open
> or short in the circuit between the O2 sensor and the computer. Bank 1
> Sensor 2 is the sensor located after the cat. If you have a single
> exhaust and cat., it is the only sensor. If you have 2 exhausts/cats, it
> is on the side that contains cylinder #1.
> --
>
> Ray O
> (correct punctuation to reply)
>[/color]