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Originally Posted by stateofidleness
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Similar to the 3.2. engine, the 2.7 variant also utilizes two sets of cats. First, there's what's referred to as the pre-cats which are incorporated into the headers . Next, there are secondary cats that are located downstream - just before the cat back exhaust. The secondary cats can easily be by-passed without any concerns relative to the vehicle's ecu . However, removing the header/pre-cat setup requires relocating O2 sensors and possibly re-programming the ecu in order to avoid cel's. Both of these approaches have been widely commented upon, along with some highly polarized views as to the effectiveness of either modification .