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Question about aftermarket cats, mid-pipe, and O2 sensors


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Apologies for making my first post a whiny, help me post.

I need new cats and don't want to fork over $3,500 for OEM parts. There are a handful of aftermarket manufacturers making mid-pipes with 200 cell sport cats for ~$1,500. Nearly all of them move the after-cat O2 sensor about 10 inches further away from the original position. The lead wire to connect the after-cat O2 sensor does not stretch an extra 10 inches.

I called two of the mfg's and asked how to solve this. They both said to switch the sides of the connections for the after-cat O2 sensor. This means the drivers side after-cat O2 sensor wire is connected to the passenger side mid-pipe and vice versa. I really hope that makes sense.

The muffler shop said emphatically, don't switch sides, citing that it can cause bad readings and possibly harm the engine. The mfg's say their customers do it all the time with no issues. I'm lost. Your input is greatly appreciated.

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First, a couple of questions: Why do you believe you need to replace the cats? In all the years of working on this marquee, cat replacement has to be one of the least often occurring repairs. Second, will the aftermarket cats pass your state’s rather draconian inspection requirements? Last time I checked, a visual inspection for any non factory emissions gear is part of the process, so before spending any money, I’d be sure you are not going to be doing this twice to get inspected…………

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I've read that tracked 996's often have the original cats die a rattling death. The honeycomb falls apart with the increased heat and stress.

The smog check is a big consideration. The mfg's state that the products are 50 state emissions legal. They do not mention anything about passing a visual inspection.

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If the cats are that bad, you should be able to see it in a scan of the engine; a lot of 996 so-called "cat rattles" have turned out to actually be coming from other sources like heat shielding. I would also move cautiously on what the manufacturer claims for their product’s CA legality.

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I wouldn't switch sides. I've gone through 5 sets of catalytic converters (140k miles) and the current set is bad. I'm about to take a spare bad set I have and have the cats welded off and a universal cat welded in place with disconnect clamps.

Good luck!

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I wouldn't switch sides. I've gone through 5 sets of catalytic converters (140k miles) and the current set is bad.

"5 sets of cats"? I don't think we have replaced that many cats in total on all the cars we service combined over the years.......... What in the Hell are you doing (just out of curiosity)?

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I have no idea. Everytime I go to the track, a week later, I hear the death rattle. And a CEL follows indicating catalytic failure. The honeycomb structure is not only cracked, but the heat generated has parts missing.

Apparently, this isn't an unknown. My friend's 3.4 Spec 996 on the other hand (when required to run cats due to sound restrictions at Laguna) does not kill the catalytic converters. In fact, He's borrowed them off my car on one occasion.

The only thing I can think of is that the car runs rich at full throttle, causing excessive heat to build up at the exhaust.

I have 140k miles on the odometer now.

I wouldn't switch sides. I've gone through 5 sets of catalytic converters (140k miles) and the current set is bad.

"5 sets of cats"? I don't think we have replaced that many cats in total on all the cars we service combined over the years.......... What in the Hell are you doing (just out of curiosity)?

Edited by jyoteen
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Yes, they're all OEM. The first 3 times were covered under the emissions warranty and then I just got used OEM cats from rennlisters, etc.

This time around though, unless someone has the OEM for sale for cheap, i'm really considering using a generic catalytic convertor due to to cost. I have until next year because of the CA smog check cycle. The car runs fine though but I have the rattle and the CEL currently, so I kow what it is.

My theory also has to do with the crappy CA 91 octane though but that's just a guess on my part. I'm convinced that 91 isn't really enough (it's borderline). I realize that Porsche tests the cars with this octane, but any prolonged full throttle driving in hot conditions and the 91 octane can't be enough.

Jyoteen - did you replace your cats with OEM the first few times?

Edited by jyoteen
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I found some used cats on ebay, but seems pointless to buy OEM cats that will fail next time I go to the track (in 2 weeks). The other options suck too - buy new OEM's for $3,400 that will probably fail or buy aftermarkets that require switching sides for placement of the second O2 sensor.

Maybe the solution is to buy a 991.

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I have had a set replaced under warranty as well (different sides replaced at different times). Rattle came after a track day both times. Since my emissions warranty is about up I am probably just going to switch over to a set of cat delete pipes for track days (use 02 sensor spacers so CEL does not come on). When my cats died I got a CEL and the durametric scan confirmed it as the catalytic converter. Are the european cats more durable? I know they don't meet USA emissions requirements but I am sure the are much more durable to live through high speed continuous use and are better than a delete pipe.

As for the conectors I suppose you could break out the soldering iron and extend the wires....

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I have a c4s and after a DE, my cats were rattling...actually it sounded like valves were tapping, but only at idle...I was worried after replacing them and all is well. Did another DE since then, and so far it's okay.

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