All standard Axle-Backs may be used on NPP equipped camaros without causing a CEL. Variable Sound NPP Valve Axle-Back (Part # 14789) can only be used on camaros with NPP from factory.
The CORSA Double X-Pipe (Part # 14788) when paired with an Axle-Back will increase the sound level of the overall Cat-Back system.
The Sport Axle-Back + X-Pipe = an Xtreme Cat-Back.
The Xtreme Axle-Back + X-Pipe = an Xtreme+ Cat-Back.
*NPP Valve Axle-Back (Part # 14789) is similar to Xtreme with valves open, Sport when closed.