Yeah when i bought my jeep it sounded a bit noisy and had my suspicions the cat was not right, but there was no lights showing on the dash, when it came to the mot a few months later it failed on the emissions i asked if the cat was bad but they weren't sure but told me the sensers were ok as they were getting readings on the computer. I bought a new cat to fit when i removed the old one it was empty nothing inside!! fitted the new one and it passed no problems, it runs better to!
straight through cat CO 0.46
new cat CO 0.01