1999 Audi A4 Check Engine light

1999 Audi A4 6 cyl All Wheel Drive Automatic

My son's Audi recently had the engine replaced with a used 2.8 (same as it had before) 66,000 miles (same as it had before). After having it for one day the check engine light came on and we took it back and they said the it read " something about the catalyst bank 1".
Well, we had that replaced and have had it for a day and the check engine light came on again. So we are going to take it back tomorrow. If it reads catalyst Bank 1 or Bank 2 being bad. Does that mean that something could be coming from the engine to damage the Catalyst? We already have $3,600 in the motor and $265 for the universal catalyst.

Thanks for your help.