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Old 04-28-2014, 07:47 PM   #1
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Replace Catalytic Converter or Entire Exhaust System?

I own a 2004 Boxster S with just over 100,000 miles. The check engine light recently came on. I took it to my garage, which Iíve been going to for almost 10 years, they indicated that they read fault codes that indicated a bad O2 sensor and Catalytic Convert. They replaced the O2 sensor and the light came back on. I purchased a cheap OBD2 scanner and it indicates a P0420 fault code, Catalytic Converter problem. I've been ignoring the check engine light because my annual inspection is not due for a couple months. I had the car in for an oil change and more diagnostic today. They indicated that I needed a new cat and that the part would cost $1,900. They indicated that they only recommend installing Porsche manufactured catalytic converters. This price seems high when I compare it to purchasing an entirely new exhaust system from Fabspeed for under $4,000. I also priced the catalytic convert out from Discount Converter LTC (www.dicountconverter.com) for between $400 and $450, depending which one is causing the problem. Has anyone had experience with aftermarket catalytic converters? Please let me know if you have any recommendations on aftermarket catalytic converters or performance exhaust system. Any information would be greatly appreciated.
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Old 04-29-2014, 10:46 AM   #2
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I would expect to lose some performance going to a cheap aftermarket converter. Installing just the Fabspeed converters should cure your problems & increase performance. You can buy a used OEM converter for the bad side for $200 that I would prefer over trying a new Discount Converter altogh I have no actual experience with them. You need to check your local smog laws also.
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Old 04-29-2014, 11:36 AM   #3
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Easy fix: Buy a used Boxster cat for $150. Job done.
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Old 04-29-2014, 11:49 AM   #4
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I agree... buy a used low mileage cat.
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Old 04-30-2014, 11:59 AM   #5
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Codes are often set downstream of the actual problem.

Unless the CAT is broken up and rattling it would be the last thing I'd suspect.

A CAT can't do it's job if the "mixture" it receives is off. When it can't do its job the code sets, but the problem is the mixture, not the CAT.
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Old 04-30-2014, 12:38 PM   #6
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Easy to diagnose... get a emission reading from a smog machine.

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Old 04-30-2014, 12:44 PM   #7
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Quote:
Originally Posted by thstone View Post
Easy fix: Buy a used Boxster cat for $150. Job done.
This^^^^^^^^^^^^^^^^^^^^^^^^^^

Which side is bad? I think I have a passenger side Cat available from the used set that I bought.
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Old 04-30-2014, 12:50 PM   #8
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Quote:
Originally Posted by lkchris View Post
Codes are often set downstream of the actual problem.

Unless the CAT is broken up and rattling it would be the last thing I'd suspect.

A CAT can't do it's job if the "mixture" it receives is off. When it can't do its job the code sets, but the problem is the mixture, not the CAT.
Really? I've seen plenty of Cats that cease to function without rattling. I destroyed the Cat on a Lex LS 400 limping home on 4cyl. That car had 1 coil per bank and when I lost a coil the raw fuel fried the Cat. Didn't rattle a bit. Bad cats on the 3.2 with <100,000miles is not terribly uncommon in my experience.


As an aside, if you had under 80,000 miles, I think the manufacturer would have to replace under warranty.
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Old 04-30-2014, 06:10 PM   #9
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Cat Replacement

My experience not too long ago:
http://986forum.com/forums/general-discussions/47002-calling-986-forum-master-mechanics.html
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Old 05-10-2014, 06:50 AM   #10
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Guys thanks for the help! I did some more research on the Discount Converter Cat and they indicated that they do not put as much precious metals in it as the OEM and that the guts are warranted for 25,000 miles. Their customer service was very nice and helpful. Not sure what the OEM is warranted for... At this price I could replace them 4 times and still not exceed the price of one OEM... Seems a little strange...
Now I've got a couple more questions. Sorry if some of these questions are element, but Iím new to working on this car. Iíve done maintenance on my Audi in the past, and have a warranty on the Boxster that just ran out. So itís time to start getting my hands dirty.
I believe it's bank 1 that it bad because it is the P0420 cel. Can anyone confirm this and if bank 1 is on the passengers side?
Where can I purchase a used Cat?
Since the car has 4 O2 Sensors, 2 on each side that are located before and after the front cats, why do I need the second set of cats? Can I simply remove them and replace them with straight pipes to the muffler? Will this cause another fault? Not planning on doing this now, but am considering upgrading the exhaust in the future.
Is there a way to modulate the appropriate signal at that the second set of o2 sensor that will eliminate the emissions issues entirely? Obviously would only do this if I were going to track the carÖ
Any information would be greatly appreciated.
Thanks again for your help!
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Old 10-15-2014, 08:24 AM   #11
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Not sure if you ever got this sorted out but I have a set off a '99 boxster that would fit through 2004 model year. $150 for the pair, would come with a few spare o2 sensors bits as well
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Old 10-15-2014, 11:29 AM   #12
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Originally Posted by justin-in-athens View Post
Not sure if you ever got this sorted out but I have a set off a '99 boxster that would fit through 2004 model year. $150 for the pair, would come with a few spare o2 sensors bits as well
Inlets are smaller than "S" exhaust so will not bolt together!
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Old 10-15-2014, 11:53 AM   #13
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I have a catalytic converter on the passenger side, I checked which a while back and if I remember correctly it is the secondary cat, so I could replace it with any used boxster S secondary cat? How much would one go for, for the sake of only doing one side.
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Old 04-04-2015, 03:04 AM   #14
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Hey justin-in-athens,

Was wondering if you still have those Cats for sale. I would be interested in purchasing them from you. Sent you a Private Message as well. Hope to hear from you. -Thanks Brother
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Old 04-04-2015, 03:58 AM   #15
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The secondary cats, which are the only cats up through 99, are not monitored on a 2000 and up 986 and can safely be removed. They are not monitored by the ODB II system in the car. (If your state has inspections you may not be able to remove them, fir off road use only, yada yada yada)

Removing or replacing those second cats will not cause or fix any p0420/0430 code.

Read Jager's thread above to.learn more about the cats and what happens when they get to the end of their life.

Last edited by steved0x; 04-04-2015 at 05:42 PM.
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Old 04-04-2015, 04:59 AM   #16
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Sold sorry
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