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Hello and welcome to just answer and allowing me to assist you. by disconnecting the cat it would throw readings off from the secondary o2 sensors telling your ECU wrong settings or if they disconnected them then it would go into fail safe , either way not what you want for proper performance. If it has no codes then may be mechanical such as compression clogged injectors , vacuum leaks etc. how long ago has it had a tune up? these are things you need to check first to eliminate the guess work. Hope this helps give you direction to solve your problem and if you require further assistance I;ll be happy to do so.