Hi there mate i have a uk 2002 wrx and fitted a de-cat 2 1/2 inch from turbo back removing both cats and never had any problems with engine management light for 2 months before it went for remap. Changed filter to a cone and that put my light on,then it went back off itself but changed back to a k&n panel before it went for map. Think you need a map to get the full benefit of fitting uprated parts though. Hope this helps you
geo