On RepairPal's catalytic converter page, it says the dealership/shop that says my converter has failed should also find out why it failed b/c the root of the problem is something else. Well, the dealership says this isn't true, there's no way to find out why it failed (unless the car shows a diagnostic code for another failed part), and that early failure (i.e. less than five yrs) is generally due to use of an after-market (i.e. non-Toyota) part or neglect of the vehicle. They said there's nothing else I could do to prevent future failure of the converter. Who's correct?