i have a 99' honda civic with two O2 sensors and i replaced my cat with a straight pipe/test pipe and now one of my O2 sensor's (the one on the cat) doesn't fit with the test pipe and it's bolted on to the frame now (open to the air). the other O2 sensor is on my exhaust manifold. will that affect the fuel/air mixture? will the straight pipe mess up my engine? would i be able to find a smaller O2 sensor to fit correctly? if i do find one... will it affect the performance of engine?

 

Most likely you just picked up the wrong test pipe honda has made many changes
to there O2 sensor set up in the last few years. when i was running my test pipe
i also had problems with it and had to plug one of the holes due to i only had one sensor. The best way to go is just get a high flow cat and avoid all the hassle. With the high flow cat you are still getting a higher flow rate while still being able to pass smog.