Cat and sensor

[T.Hanson]

What improvements come with a new cat and oxygen sensor ?

Old cat buzzes.

I.e., can those components be suspects in hesitant acceleration, trailer hitching, bogging. Very minor, but annoying after tuning everything else twice.

[Freddy]

Well, I got a new Catalytic Converter about 1 year ago. Mine was rattling. I have seen this a few times before. You said "buzzing", so I'm not sure about that sound. But, the catalyst is kind of like a ceramic honeycomb and is held in place by some insulation that is sort of like steel wool or something like that. This material that holds the ceramic core in place can break down and get blown out. Then the core rattles around and can break apart. It is possible for the pieces to block or restrict the exit of the converter, and you can imagine what problems this could cause. If your oxygen sensor fails and the engine runs too rich for a time, this can cause the converter to get too hot and start to destruct. The O2 sensor is critical to the proper operation and long life of the cat, so replacing them at the same time is a good idea. I got an aftermarket converter at a local muffler shop and the bill was around $275, but I also believe that the unit they installed may not have as long a life as the original. Personally, I'm OK with that because the original may have lasted 100k - don't really know. Maybe one of you guys in California could speak to how long they last since you get emissions testing every year and could track its performance. There is no emissions testing where I live in Florida. Hope this is helpful to you. -F

[Lenny D.]

Once again Freddy is right although I don't subscribe to the idea of replacing a cat every 30k when the O2 sensor is scheduled to be replaced. An original BMW cat lasts much longer than that. Having said that an aftermarket cat. would likely not make 100k and beyond, so more frequent replacement invites thought.

To answer T.'s question, minimum emissions as originally engineered and stoichiometric combustion are the improvements that come with cat. replacement and O2 sensor respectively .

The 1k work dept. stikes again, courtesy of Bosch L-Jet Technical Instructions booklet making F's words visible:

[Mike W.]

OE cats are a wonder. My first E12 passed comfortably with the original cat at 347K, although that was before they tested for Nox. My recently sold E28 535 passed with flying colors at 330K, and even Nox at a 295K test. (Nox testing isn't required where I now live) Aftermarket ones, a year, maybe 2. The OBDII ones now required in California may do better, if you can find one for an E12. But good cats just don't wear out. I would do a whole lot of welding if needed to fix an OE cat if the shell was the only problem.

[Freddy]

Wow, Mike. That is very interesting to me. Having not lived in an emissions testing area, I really had no idea how long those things could last. We would replace them usually only in cases of extreme obvious failures that likely happen when people ignore other emissions related problems on older cars because they can. My car had about 225,000 miles when I replaced the converter. It looked like the original and I wondered how likely this was. It may have lasted longer, but the car was very poorly maintained before I bought it and rebuilt the engine. Before, it burned sooo much oil and had lots of problems with fuel and ignition. I expect this was very unhealthy for the cat. I did notice with the new cat that the exhaust smelled very different - cleaner I guess. And Lenny, I do agree that periodic replacement of the converter at o2 replacement intervals is really not necessary. I see that what I wrote wasn't really clear. More accurately, I think that in regards to the expense of replacing the cat, another $20 for a new sensor is just good insurance unless fast switching of an existing sensor was verified. And gosh I hope the one I bought will last at least 50,000 miles