Welcome. It seems only the GM converters from 01-02 GP's with the 3.8 seem to clog, and Id have a hard time believing that any aftermarket converter would clog. Ask him how he knows yours is aftermarket, and let him know that the cat warranty is extended to 10 years, 120,000 miles, regardless of owner. If all else fails, take it to another GM dealership. I just had mine changed out last week because my cat was clogged, and the car wouldnt go over 15mph. The previous owner thought it was the trans that was bad haha.