As mentioned, the 2 front 02 sensors adjust fuel trim. The 2 rear 02 sensors, basically let the ECM know the cats are working by comparing the 02 level before and after the cats. Yes this can be tuned out, also there are simulators that can be installed. There are 2 types. One is done with an electrical resistors and other electronic parts. Youtube has videos on making them. And there is a simulator made from 4 spark plug nin fouler spacers with a little piece of the honey comb material that is inside of the cat it self. You have to cut and grind the honey comb to fit inside of the non fouler. Use a mask when grinding, along with eye protection. I made a set of the simulators when I gutted my cats with that material from the cats and the spark plug spacers. They have been on my truck for several years, never a CEL. It works better with 2 different size non foulers. The longer one goes closest to the cat with the honey comb inside. Here are a few pics of mine.
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