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I have a '99 Honda Civic Si, and I recently put on new Comtech header, Catback Exhaust, and a cheap EBay high flow catalytic converter (off road use only). In a 100 miles, my check engine light came on, possibly indicating that my emissions are off according to my owner's manual. Could the signal from the new catalytic converter be doing this or is something else wrong?
Any advice or info would be much appreciated. Thanks.

 
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take it to a parts house and they should be able to runa diagnostic for free.... i know auto zone does that... but 100 miles after a "high flow cat" install from ebay sounds about right... that's probably all it is...

 
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does the cat take the o2sensor???????? if the sensor was not installed then that is why your check engine light is on.....

 
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any half descent muffler shop would re-install an o2 sensor... it doesn't go in the cat either.. well, it may be built like that on some cars but you have one pre-cat and one post-cat... it doesn't have to go IN the cat

 
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on an 99-00 si they are part of the cat ... there are not a lot of companys that make a free flowing cat for a 99-00 si because of that reason

 
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I had diagnostics done on my car and it came back as low back pressure in the catalytic converter. Is there any risk to my engine w/ this, or is it just because the oxygen sensors are set to a factory setting for the old cat?

 
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I have a 2000 SiR adn the same thing happened. Mine came on after installing a new cat. The dealership said it shouldn't cause me any grief but the car might start running a little rough. I have had the cat and the light on now for over a year with no problems.