You can get cats for cheap and if they are engineered right you won't throw a cel. I ran GTM unreasonated test pipes, no tune but the sensors are set where no CEL came on.
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2009 Carbon Fiber/Red 370z aka Trish: Sports Package, Meisterschaft full catback, Fast Intentions Longtube Headers, Injen Long Tube Intakes, UpRev Tune: Sold 
2008 Lotus Exige S Club Racer aka Bella, always leaves me smiling
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