01 Pontiac Grand Am service engine light catalytic converter?

I have an 01 pontiac grand am se 2.4L 4 cyl. automatic. the service engine soon light is on. the code says catalytic inefficiency or catalytic efficiency low. the car has an o2 sensor in front and behind the cat. what can i do to trick the o2 sensor? or the better option, if i put an after market cat in will the check engine light or service engine light stay on or off??