I've seen the TSB for warranty cancellation, and every time I think of it I almost laugh because let's be real: The fact the driver has an FFS, or a Mustang, or any kind of performance car like that, is a "sign that the owner may have done other modifications", if you think about it. But seriously, I think the TSB actually mentions things like racing bumper stickers even as cause for suspicion.It's not that a catch can makes the emissions system less effectivce, because it doesn't. The dealers (or Ford with warranty work) are more worried about owners creating system leaks, improper flow, etc. that could cause other damage or issues. It's also one of those things that is a "sign that the owner may have done other modifications".