they shouldnt void the warranty. just tell them that you have a cel from the dp. be honest. they will clear the code for you once and then after that they will charge you. i never had any problems with them about the cats. i actually did a fix for the cel after it came back on again. i ordered a piece from rallysportdirect and it has worked ever since. this is what you want to get:
RallySportDirect.com

go here buy that. i would have to show you how to set the clearing up once it is on the car. its not hard.