So I have read every post in this thread and now feel compelled to chime in.
In stock configuration you have two pre cats and two main cats. Now, you have a driver side unit and a passenger side unit. Each unit consists of 1 pre cat, 1 O2 sensor and 1 main cat. That O2 sensor is located directly between the pre cat and main cat. It is considered your rear O2 sensor. It is strictly for emissions and does nothing to your Air/Fuel ratios. Your front O2 sensors which are located in your exhaust manifolds control your Air/Fuel ratios. Most people commonly refer to the exhaust manifold as a header.
By going to an aftermarket catalytic converter you will eliminate your pre cats! If the anti fouler on your aftermarket cats are done correctly, you should not get a CEL. To also ensure you never get a CEL, you need to make sure there is no exhaust leaks. Specifically between the exhaust manifold and aftermarket cats.
If you hear a hiss under acceleration you most likely have an exhaust leak. It could be anywhere and what you are hearing is a fluttering sound. If you drive between to large objects, buildings walls, etc... it will be most prominent! The "hiss" under deceleration is completely normal. It is at it's loudest when most or all of the back pressure is relieved.
I think that about sums it up. If you have any questions, please do not hesitate to give me a call.
Thank you, Tony
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