well back when i had my vw gti and the cat went out i had little pieces of ceramic i think it was shooting out the tailpipe...performance would bog down for a time til whatever blockage inside would be blown out the back
u can also check your muffler...tap it on the sides with your hand a few times if its wearing out u can hear the 'looseness' of the material inside...maybe this same way can be used to check your cat
mind u, this is from experience long time ago with a vw gti
