On an 02 996, there is a rattle coming from the rear that sounds like the left side; potential etiologies are: catalytic converter, muffler, and muffler tip. There is no Check Engine Light; car passes smog with no problem. I have two questions for you all as I rule out the catalytic converter as the cause:

1. If a catalytic converter is bad, and if it is removed, should I be able to reproduce the rattling noise by shaking it, banging on it, "clapping" over the pipe end to simulate pressure, or does this rattling sound only happen with the air pressure of the exhaust?

2. Have any of you guys who have had a rattling catalytic converter remove it and actually be able to rattle it off of the car?

This car is not covered by the dealer... so I am the one doing the fixing! Thanks for the help.

 

just hit the cat with a closed fist. It will rattle on or off the car. If your car is under 80k it's covered under warranty.

 

Ok... so if that's the case, my catalytic converter is fine since I can't get it to rattle in any way - hitting it with a hammer, fist, nothing... I was not sure how easy or possible it was to get it to rattle off the car. Lots of people talk about replacing them via the dealer when they hear a rattle, but I can't find a mention if it rattles off the car!

Thanks for the response. Back to re-assembly!

 

Ok... my cat was the cause of the problem. By looking inside, I could see that the forward-most section of the cat internals had slid forward about 1 inch, and was actually crushed into the middle oxygen sensor. I removed the O2 sensor, and was able to slide the core back into place. Since the cat has multiple metal layers inside of it, I was able to reach through the O2 sensor hole and crush down the perimeter 1/3 of the innermost layer to "crimp" the ceramic core into place. Now it doesn't slide forward anymore, and has a sizable crimp to overcome before it gets dislodged again.
So I learned that my cat did not rattle when hitting it or shaking it, because the core was intact, but had just slid forward into the larger section and was rattling around as a result, partly pinned by the oxygen sensor. Looking into the shorter end and the O2 sensor hole revealed the problem. Based on how much I crimped inwards the internals, I can't see how it would happen again, but we'll see!

 

I have had 5 cats replaced in my 996, fortunately all under warranty.

 

same here, it will be expensive to replace once they are not under warranty. Why do they make such a inferior product.

 

cats mostly fail due to something wrong with the car. They do not fail.

 

That's a lot of cats. Here are a few questions:
- Do you track the car(s)?
- Did the cats all fail the same way?
- What was (were) the failure mode(s)?
- Did both cats fail simulatneously or just one at a time?

Like JHoffman said, a failed cat is usually a symptom of something else going on.

 

i'm on cat set #3. Latest being replaced in April or so. I hope the new one comes with at least a bit of warranty since it was covered by PCNA.

 

Daily driver 96k km in 5 1/2 years
Tracked for 3 years but replaced a first set during the first 2 years of not tracking.

They failed all in the same way - Started hearing rattling from the exhaust, then after some time not always right away, the check engine light would come on. Reset it and sometimes would take a few days or weeks for the check engine light to come on

Both did not fail at the same time, one then the other one a few months later.

 

My first one failed last Fall with 35k miles, 2nd one this Spring with 40k miles. Same symptoms.

Fortunately, warranty for me - but I like the OP's solution. Other option is cat bypass pipes. I have a set, but need to get around to trying them out.

 

Rattle question--if they are bad, do they rattle at idle or just over bumps. Thanks!

 

They rattle all the time though once moving you can't hear a rattle.

 

I did my cats (under warranty) last year.

I do believe that the sulfur content of texas crude based fuels (sour vs sweet) has some impact on cats, might help explain why a few people i know down there have had similar issues

In my case, the cause of the cat damage was faulty coil packs causing misfires, also covered under warranty.

I am the sort of guy who will run without cats if they fail outside of warranty, they are a fortune on all german cars

 

Sounds like multiple failure modes.

Modern cats are monolithic "honeycomb" assemblies. They can break, and they can come loose inside the can. Repeated thermal shock can cause enough distortion in some cats to physically break the substrate (think running off the track backwards into a snowbank or a water puddle).

The substrate can melt due to high temperatures caused by excessively rich operation (raw fuel getting to the cat). Misfire results in rich air-fuel ratio. A melted cat will often be accompanied by a flow restriction that bogs the engine.

Excessive sulphur in the fuel can make a cat go bad. So can leaded gas (even one tankful).

Silicone sealants or engine coolant that gets into the exhaust can also destroy catalyst efficiency.

I asked the question about tracking the cars with failed cats because I'm searching for a track solution. Tracking the car undoubtedly reduces cat life. It appears that the only realistic solution may be to run with cat bypass pipes, but the accompanying CEL might mask another problem.