GMC Troubleshooting Questions? Ask a Mechanic for Answers
Yes, theoretically the cat could be good and the code could have been set by the downstream sensor sending a signal similar to that of the upstream. However, for that to happen, air would have to be entering the exhaust stream between the cat and that sensor. In conclusion, unless there is a hole in the exhaust pipe, the cat is no longer doing what is supposed to do and needs to be replaced.
Please let me know if you have any other questions.