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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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