my girlfriend has a 1998 civic ex, the car has been nothing but problems, the exhaust system had a giant hole in it, so ichanged it, but the stock cat was fused onto the system, i put a cheapo obx high flow on it, it has been throwing the o2 sensor code. i put a new one in two days ago and it worked good, but now the code is back. anyone have any ideas?

 

If it is an O2 sensor code, you have a problem with your O2, check the sensor and see if you may have frayed a wire or nicked it during replacement of the cat. If there are no obvious problems with wiring, I would suggest a replacement.

What code did you throw exactly?

 

That OBX "cat" is likely nothing more than a resonated test pipe, in which case the secondary O2 is going to recognise that there's no functioning cat, and throw a code.

 

02 sensor will also throw a code if your emmisions are much like your primary 02 sensor's reading, which would indicate to your computer that your cat is deficient and worn out. That may be the case with your highflow cat, if its not street-legal and filtering your exhaust correctly.
This type of problem can be solved by placing a gapper on the 02 bung, i know BLOX makes a version. This gapper pulls the 02 sens farther from the pipe and exhaust flow, so it reads less emissions, thus eliminating your CEL. This is for test pipe setups tho, and really serves no purpose other than to eliminate the CEL while operating the vehicle. These things wouldnt help you in the least for an emissions test at inspection, despite if you are OBD2, even in NJ they still stick a sensor in your exhaust pipe.