The true dual systems like those from AWE or Borla require cutting out the crossover pipe that
@AXE showed. They clamp directly behind the cats.
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The typical Catback systems for these trucks use the factory union and is effectively a dual, to single, to dual exhaust. Stupid design if you ask me but once again it’s for packaging reasons from the OEM.
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The midpipe itself is a an emissions component. While the physical cats are not modified, the part itself is.
A shitty dealer would have the ability to nullify warranty because of it. Very unlikely, but that’s how the manufacturer warranty is written.