"code shows excessive back pressure"
The code saying excessive back pressure means nothing without proper diagnosis.
Did they actually check the back pressure??
The computer uses the value difference between the front and rear O2 sensors to determine back pressure, so if the cat is plugged, you have another problem somewhere else, causing the problem.
We need to know what the EXACT fault code is, to be able to be of any help.
What year, make, model and engine???