So I took my truck to my mechanic today to check why my check engine light came on, they said that the Catalytic converter was bad. I told them that the car dealership that I bought this truck from replaced that. They told me to contact the dealership and I did. Have an appointment in the morning to have them check it also. Now I need to know( what could be causing the Catalytic Converter to be going bad? ) My mechanic didn't check for the problem so I will have the dealership do that. any Ideas on what could cause this problem?

your trucks cat may still be under warrantee and the dealer just told you they replaced it and didnt to buy time. usually you can tel if its been replaced or not on a newer vehicle because it will look cleaner or less rusty than everything else.

http://www.allpar.com/dealers/warranty.html

but to answer your question, misfires or over fuel will accelerate cat deterioration