I really hope I don't have to live with engine fault lights?
You will with a downpipe unless you get a defowler or tune it out. I am guessing you are on stock tune. To put it in simpler terms; Your ECU is programmed with the stock cat and measures the air quality coming out the back end of it via the o2 sensor. If there are too many nastys coming through it will think your cat has malfunctioned and throw a code. Because you installed a high flow cat, there are more nastys getting registered by your o2 sensor.
There are 2 ways to combat this:
1. Get a defowler, google it. It is a J shaped piece that moves your o2 sensor out of the exhaust stream. These is the cheapest route.
2 Get a tune to remove the code. You can go to a tuner and they will be able to remove the code for you for good. If you are going to get a tune anyway than don't bother with the defolwer and just do this. Otherwise it's not worth it to go through getting a tune just to remove your CEL..
