No you can't use the O2 sensors to judge a cat
Posted to Emissions Forum on 1/15/2011
81 Replies
The recent thread where I had my fun at Lances expense needs
some closure. First, when encountering a catalyst code there
are a few things that the technician must confirm. They are
in no particular order that there are simply no TSB's or
software updates. If the engineers got the testing wrong and
the catalyst is failing falsely, or prematurely as Rags said
that is where you will have your only chance to do something
about it. If the engineers have not released an update to
fix it, you don't have a chance to prove or disprove the
catalysts ability to do its job, because as was said more
times than I want to try and count, you'll still have to
trust the PCM.
The next thing for the technician to check and rule out is
exhaust leaks, and the performance of the O2 sensors.
Exhaust leaks are pretty straight forward, they rarely play
"Hide and Go Seek". Keep in mind, an O2 sensor that is
marginal may pass the PCM's test of it, and then turn around
and not be able to accurately report the exhaust content and
trick the PCM into a false result. At the same time keep in
mind for that to result in a false catalyst failure, it
would have to occur two times in a row. That fact alone
reduces the chances of that occurring, but does not
eliminate the possibility.
The technician must verify good fuel control, and fuel trim.
Exhaust leaks obviously have a potential impact on fuel
trim, but so do certain input signals that the PCM relies on
to recognize when its inside or outside of the designed
testing window. Different manufacturers use different
routines to establish their testing thresholds, GM typically
tests at idle and they don't even have to of completed their
O2 sensor monitors. If by chance one of the O2 monitors fail
after the catalyst monitor has run, the decision could be
made by the PCM to suspend the result depending on the
results of the next test. Ford on the other hand runs the O2
sensor monitors, and then completes the catalyst testing
under varying light load conditions. If the O2 sensor
monitors do not complete, it should be expected that the
Ford system will not even attempt to run the catalyst
monitor. BTW, do leave room for exceptions as neither of
these are hard facts, just tendencies. Always refer to
service information for specific details on any vehicle you
need to diagnose.
One of the biggest issues with P0420, and P0430 codes has to
do with investigating why the catalyst failed in the first
place. We can argue this one till the end of time but it's
my opinion that far more catalysts are killed by external
causes than simply die of old age. Sure both types of
catalyst death occur and I have no statistics to prove my
assertion that converters are more commonly killed than get
to live to old age. But in a limited study of two vehicles
to support my take on the matter I do have data that I
collected from have two rather common vehicles, a 1999 F150
with 287K miles, and a 2002 Ford Explorer with 143K that
both have all of their original catalysts on them and all of
them are still going strong.
[1999 Ford F-150, Emissions Scan Data]
[2002 Ford Explorer XLT, Emissions Scan Data]
Now when you look at those captures for the catalyst you see
the pids called rear to front switch ratio. Things like
that, and descriptions from many manufacturers about how
they compare the O2 sensors outputs during the onboard
testing of the catalysts is what I believe leads to the
misconception that technicians in the field should be able
to evaluate a catalyst by watching the sensors. Let's see if
we can explain why it just won't work for us, and it's not
about what we can see which are the O2 sensor signals, its
all about what we can't see and that is exactly how
efficient the catalyst should be right at the time the
technician is looking at the data.
There are a lot of things to consider about a catalyst and
how efficient it should be. Of course the age of the
assembly is important, and when it comes to replacement
catalysts, construction and its size (the volume of and the
percentage of the elements inside it) are critical. But for
now, lets concentrate on just the O.E. version and what we
can ascertain. When the engineers set up to test a catalysts
efficiency and of course to decide when it is no longer
capable of doing the job that it has been assigned they took
on the task of trying to predict what the converter would do
under a specific range of conditions. To do that they rely
on a process called modeling. The short and not perfectly
accurate explanation of modeling would be where they first
had to predict if the vehicle was driven under specific
conditions that the catalyst would be a specific
temperature. Then with a given airflow (engine load), fuel
trim, engine rpm, and the heck if I know what all else, the
locus length of the upstream sensor would be XX times longer
than that of the downstream one.
Locus length is pretty well defined by another member of the
iATN as the length of a piece of a string that if you took a
specific period of time and graphed out the O2 sensor, you
could lay that string on top of the graph and measure its
length. You would then repeat that for the down stream
sensor and measure that string for the same period of time,
and that ratio between them is the answer to how well the
catalyst worked at that moment. As a technician, you could
see sensor output that could easily be interpreted as the
converter operating, but you could also see just the
opposite result at any point in time. You have no way to
know just how efficient the converter should be right now,
if it is even lit off at all. Even worse, you could
accidentally be looking at the sensors while the PCM is
actually running the test, and its intentionally forcing the
rear O2 sensor to move and it would easily look like a bad
catalyst to you, when the PCM happens to be ready to pass
it.
If you had a way to know exactly what the sensors outputs
should be at a given catalyst temperature, and engine load,
and every other variable that the engineers decided was
pertinent, then and only then could you try and make sense
from the sensor outputs. But even that would be only for
that one specific moment in time, and not necessarily
through the catalysts entire expected range of operation. By
modeling the engineers work to predict what the catalyst
will do under a wide range of conditions and one that (just)
passes under one specific set of circumstances may fail
under others.
So what does all of this mean? When you retrieve a
P0420/P0430, inspect for the things that you have the
ability to check, and in their absence there is nothing else
you can do but trust the PCM, and make sure to the best of
your ability that there are no conditions with the vehicle
that are likely to kill the new one.
John from Pennsylvania
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