There are two ways to tell: by trouble code and by backpressure testing. If you're saying the computer board is bad, that option is off the table for you. To test backpressure, you usually remove the post cat sensor and install a pressure gauge. Testing post cat tells you if there's a muffler restriction. Then you do same thing on pre-cat. If cat is plugged, the pressure will be much higher on the pre side than the post.