I put a header on my car and now my car idles wierd. It'll sit and idle fine for a minute, and then drop suddenly to around 200rpm for 5 seconds, then it'll go back up to normal idleing rpm. Since I have a 2002 dx civic, the cat is connected to the exhaust manifold. And when I took it off to fit the header, I now have no cat. I'm throwing an o2 sensor code but I don't want to change it until o figure this out.

 

Which sensor code? The first sensor doesn't depend on the cat. It is possible for a sensor to break when you unscrew it and try to reuse it though so you may have a bad first sensor now anyway.

Put a cat on. I don't want to breathe your pollution. With a header you use an EX type cat under the car. The wires for the second sensor at the outlet of the cat will need to be extended, they sell a plug and play harness for that.

 

Ya, that's been my plan. Do you know if they sell cats with an o2 bung attached to the end of it, or do I need to weld in a new one?

 

Stock cat or a decent brand like a Magnaflow will have the port. Avoid cheap no-name ebay stuff they are often just empty shells.