2002 Toyota Corolla after market catalytic converter

2002 Toyota Corolla Front Wheel Drive Automatic 96000 miles

I took my car to a mechanic and they diagnosed I needed a new catalytic converter. They put an after market part on my car and my check engine light still won't go off. My car has been burning alot of gas I have to fill up twice a week now and my car vibrates when I come to a stop. The mechanic told me some toyota's take the part and some don't. Is there anything I could do to fix this? They are trying to charge me even more money for the manufactorers part.