That cat is in a pain in the butt spot to replace, Ive done a few, ick. Here's the million dollar question, did your mechanic determine what made the cat fail? Is it plugged up or is it setting a P0420 cat efficiency code? P420 means its not working as designed anymore, but may not be plugged. Plugged, is well, the opposite of a japanese powerplant. If it is plugged, what made it do that, was running bad, etc? What I'm getting at is, they dont just plug up, it has recently in the past, or is recently dumping too much fuel down the exhaust, and melting it down. If the problem isnt corrected, youll toast the new cat as well, and every one u put on until its fixed.