Well you would go and buy a new fangled techno toy wouldn't you. What have you done to it?
The oxygen sensor gives information to the CPU regarding fuel burnt which will then adapt the fueling and timing and is designed so that (in theory) the engine will run optimally. If you're going to remove the cat and replace the exhaust (with a Remus full system?) then you might find that a remap will be best (if available).
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