If I'm not mistaken, it isn't necessarily indicative of failing cats. But rather, it's symptomatic of cats that are being worked very hard and possibly a lean air/fuel ratio? Someone correct me if I'm wrong.
1998 BMW 328is Shooting Brake
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
View my photos: Caught in the Wild