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RIP George...
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Discussion Starter #1
My Kooks catted LT's finally threw a code today. P0420. I noticed the smell of rotten eggs (just this morning, not any other time) as well as a decrease in my fuel mileage the last couple of times I had the car out. Do I need a new cat or just a tune?
 

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My normal state of affairs...
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5,909 Posts
rotten egg smell generally means bad cats. them being clogged could also be the reason for your MPG too.
 

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RIP George...
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Discussion Starter #3
I do want to add that the only time I had the smell was this morning. It's now gone and the mileage went back up after I filled it up this evening.
 

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Owns 3 GTO's
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16,515 Posts
You've set a cat efficiancy code. The cats provided with headers flow too well to properly moniter them. Just have it reset, and later get a dyno tune. This will remove the rear o2's from the computer.

I have the same code. Mine has been setting ever 100 miles or so. I get horrible milage till I reset the light. BTW, I do not have cats.
 

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RIP George...
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Discussion Starter #5
I can reset it with the old Interceptor scangauge. The wierd smell just threw me off. Time for a tune methinks.
 

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Owns 3 GTO's
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That smell is from the rich condition.
 

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Exhausted Tuner
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Striped04 said:
My Kooks catted LT's finally threw a code today. P0420. I noticed the smell of rotten eggs (just this morning, not any other time) as well as a decrease in my fuel mileage the last couple of times I had the car out. Do I need a new cat or just a tune?
Headers require tuning ... The cats are probably not dead, if they are melted or blown out you would hear it.
 

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RIP George...
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2,163 Posts
Discussion Starter #9
mypetgoatAZ said:
how do you get the code?
I've got a pair of Aeroforce Interceptor scangauges that read the DTC codes. They can also clear them out. The website has a guide of what the codes mean. Nice tool to have.
 

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Owns 3 GTO's
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I just use my scan tool at the shop. But, your setup is cool.
 

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Kooks

I am adding the "kooks" long tubles on my '06, should I get or do I need their "cats"; are they an easier matchup then getting magnaflow stainless steels ? Are the "kooks" cats better than stock? I have the magnaflow exhaust with "X" pipe.
 

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Owns 3 GTO's
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Most after market cats flow much better than stock. If you are asking if you can use your stock cats with LT's. The answer is no. As long as the connection pipes/cat pipes are install correctly. They will bolt up to any well designed cat back exhaust. You should have a tune done if you get there cats. You WILL need a tune done if you go with offroad pipes.
 

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Ka-Boom!
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1,987 Posts
I was getting that smell when I put on headers witrh cats. It seemed to be worse on a cold start up, probably due to still being in closed loop. After the tune I havent noticed any foul smells. I also never got the CEL either, the Tuner did turn off the rear O2's because he said it was a matter of time before I got a light. I think a tune is a must for headers and I feel it was worth it, it runs much better.
 
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